The galaxy was one of many in the universe. Much of the galaxy was once ruled for a thousand generations by the Galactic Republic, but fifteen years before Star Wars Rebels Season 1 the Republic was transformed into the tyrannical Empire following the end of the Clone Wars.
The galaxy was also home to two major Force-using orders: the Jedi who championed the light side, and the Sith who championed the dark side. Yet another order included The Bendu, who described himself as "the one in the middle."
History
Succinctly, large interstellar galactic civilizations first propagated thousands of years ago in the space sectors of the galactic Core, then radiated outwards. Interstellar civilization was made possible through the discovery, by different species, of Hyperdrive engines, allowing faster-than-light travel through hyperspace.
Stars are more densely concentrated in the core, making a concentration of resources and interstellar civilizations more likely there. The Core worlds have benefitted from being hubs of interstellar development and trade for millennia, and they contain a drastically disproportionate amount of the galaxy's population and industrial infrastructure. In time the Core worlds united as the Galactic Republic, after which colonization waves spread outwards to the rest of the galaxy, all the way to the Outer Rim at the galaxy's edge.
The capital world of the galaxy-spanning Galactic Republic for hundreds of generations was Coruscant, one of the Republic's founding worlds. After the Republic was transformed into the Empire, Coruscant continued as the capital world. Every world in the Galactic Republic was also interlinked using the galactic holonet, an information network for transmitting news and entertainment. The holonet was later strictly controlled by the Empire and largely limited to propaganda.
A few pockets of the galaxy were never part of the Republic, or even the Empire - from isolated neutral systems in the middle of Republic space to larger sub-nations spanning multiple sectors, such as Hutt space. Both the Republic and the Empire, however, controlled the overwhelming majority of the galaxy, to the point that they could justifiably be caled the Galactic Republic and the Galactic Empire.
Astrography
As a general rule, planets closer to the galactic core tended to be more developed, wealthy, and overall important, and planets closer to the Outer Rim tend to be less so - backwaters relatively undeveloped, though still quite civilized and part of the galactic community at large.
Important Core worlds include planets such as Coruscant and Alderaan.
At the opposite physical, political, and economic extreme were modest frontier worlds in the Outer Rim, such as Lothal.
The uncharted sectors beyond the Outer Rim, at the fringe of the galactic spiral, were known as Wild Space - where the stars are scattered thinly, and the cold of space seeps in. Settlement waves from the rest of the galaxy had not yet extended to the few systems and planets in this area (what made a system "wild" was that it was uncharted and unsettled; once it was incorporated into the rest of the galactic community, it was simply considered another world in the Outer Rim). Several hidden worlds exist out in Wild Space, such as Lirasan or Kamino.
Culture
The galaxy was home to hundreds upon hundreds of different sentient species, with diverse cultures. Before the formation of the Republic, interstellar civilizations grew haphazardly, with early colonies founded by sleeper ships or generational starships; other colonies simply lost contact with their homeworlds and forgot their origins. There were many species that actually could not accurately confirm which planet was their original homeworld, the truth simply having become lost to the mists of time. Ironically, one of the most prolific species in the galaxy were humans, which was one of the species with no clearly identifiable homeworld. Many human colony worlds in ancient times were concentrated in the Core worlds, and became influential founding members of the Republic. The humans of Alderaan, for example, were aware that their ancestors were colonists from another star system, but tended to identify as "Alderaanians" first, "humans" second, and develop rivalries with other powerful human-colonized worlds such as Corellia or Coruscant. Many planets, even colonized by the same species, could subsequently develop quite distinct cultures, i.e. a human Alderaanian such as Leia Organa and a human Mandalorian such as Sabine Wren.
Nonetheless, the subsequent influence and stability of the Republic for thousands of years allowed many galaxy-wide cultural, economic, and governmental practices to develop.
Due to having one unified government ruling the overwhelming majority of the galaxy for millennia, "Galactic Basic" proliferated as primary language used throughout the galaxy. Some cultures have their own local languages (such as the Mandalorians) but also know how to converse in Basic. Certain other species were physiologically incapable of speaking Basic - such as Wookiees, Ithorians, etc. - and had their own languages, but could still learn to comprehend Basic when they heard it.
Credits were used as a standardized monetary system across the galaxy, first by the Republic and later the Empire (transforming the Republic Credit to the Imperial Credit).
Several games and sports also became popular across much of the galaxy, such as Dejarik (holochess) and Sabacc (a poker-like card game).
Behind the Scenes
The Original Trilogy established the basic astrography of the Core Worlds which are the most powerful and important, sliding on a spectrum down to the Outer Rim planets such as Tatooine which are poorer backwaters.
The original Star Wars Expanded Universe, now known as the Legends continuity, grew and evolved for over three decades from the release of the Original Trilogy films until Disney acquired LucasFilm in 2012, at which point it decided to reboot the entire Expanded Universe, to clear the way for the new Sequel Trilogy. The original Legends Expanded Universe developed extensive details about the astrography and history of the Star Wars Galaxy, but much of it has not yet been confirmed as being true for the new Canon continuity as well. Star Wars Rebels is the first major TV series set in the new Canon continuity.
Some general regions of the galaxy were described in the Legends Expanded Universe, but they have not yet been confirmed in the new Canon continuity (and thus, applying to Star Wars Rebels). Radiating outwards from the center, they included:
- Core worlds - the important worlds at the center of the galaxy which contain a disproportionate concentration of most of the galaxy's population, wealth, and infrastructure. The Core worlds united to form the Galactic Republic about 25,000 years ago.
- The Deep Core - in real life, the radiation from densely concentrated stars at the center of galaxies would actually make them uninhabitable. The original Expanded Universe therefore later made some concessions to this by explaining that the "Core worlds" like Coruscant are as close to the center of the galaxy as is safely possible, but the "Deep Core" has stars clustered so densely together that it is dangerous, the gravity distortions from high star concentrations resulting in barely any safe hyperspace routes, and the few accessible planets barely inhabitable. Many secret bases, smuggler's dens, and the like are therefore hidden in the Deep Core.
- The Colonies - the systems that the Core worlds expanded into in their first great wave of colonization thousands of years ago - so long ago that by this point the Colonies are just as powerful as their founders in the Core worlds, in some cases even more powerful. They are only called "The Colonies" as a historical artifact, and the only difference between them and "the Core Worlds" is a matter of prestige. This is comparable to how in real life countries such as the United States, Canada, or Brazil started out as "colonies" of European countries (Britain, France, Portugal, etc.) but over the centuries grew just as powerful as their founders and became independent. Their histories aren't nearly as long and prestigious as their founders, but this doesn't affect raw political and economic power. The Colonies are often thought of as just an extension of "the Core Worlds" in a broader collective sense.
- Inner Rim - originally known as just "the Rim", when it was the outer fringe of the colonization waves from the Core worlds. This was so long ago that the name "Inner Rim" is largely a historical artifact, as worlds in it are quite developed - though generally not quite as powerful as those in the Core/Colonies. They have a mid-level of importance (in terms of politics, military, population, wealth, infrastructure, etc.), and form a bridge between the Core and the less developed sectors of the Outer Rim.
- Expansion Region - After several thousand years, a new, massive colonization wave from the Republic burst out into the galaxy, radiating out from the Core and Inner Rim. The new sectors they expanded into became known as the "Expansion Region", in an era of aggressive settlement which soon turned into ruthless colonialism and exploitation dominated by major corporations and the more unscrupulous Core worlds. The whole effort was comparable to the real-life Manifest Destiny colonization wave in North America, pushing west into Native American lands, or the Scramble for Africa by European colonial powers. Over time, however, the Galactic Republic started reining in these abuses, and the Expansion Region had been settled for thousands of years by the time of the Original Trilogy. The Republic learned from these mistakes and sought greater oversight when the Outer Rim was being explored and settled.
- Mid Rim - The Mid-Rim generally didn't have the vast untapped resources of the Outer Rim, so when a new colonization wave later spread out from the Expansion Region toward the galactic rim, many colonial efforts simply skipped over the Mid Rim to reach the Outer Rim. Planets in the Mid-Rim often have to work hard to survive - though there are still many prosperous planets scattered throughout the Mid-Rim, such as Kashyyyk.
- Outer Rim - The vast outer edge of the galactic spiral, in many cases frontier planets only settled relatively recently - the Galactic Republic started expanding into the Outer Rim only about one thousand years ago (compared to the 25,000 year history of many of the Core worlds). The Outer Rim has vast untapped resources, with many worlds colonized as mining settlements or farming planets. When the Empire rose to power, however, it began to ruthlessly exploit the Outer Rim: its planets have large amounts of raw resources but are generally too weak militarily or politically to put up much resistance, and they are located far away from the prying eyes of the Core Worlds that might raise some objections. Tatooine and Lothal are located in the Outer Rim.
- Wild Space - "Wild Space" is a somewhat inconsistently used term applied to systems that haven't been formally colonized, beyond the Outer Rim. They are on the true fringe of the galaxy, in sectors where star systems are few and scattered. While they are unexplored, they are not uncharted, and a few basic hyperspace routes into the area are known (in contrast with the Unknown Regions). Basically the difference between Wild Space and the Outer Rim is that a system in Wilde Space hasn't been formally explored and colonized yet - once it has, it is then just considered part of "the Outer Rim".
- Unknown Regions - The Unknown Regions are vaguely defined as parts of the galaxy which haven't been explored or charted at all. The definition evolved over time in the original Expanded Universe, before more official maps of the galaxy were worked out. Originally the Unknown Regions were just the extreme fringes of the galaxy, on the other side from Wild Space - "Wild Space" was itself somewhat of a later term, and often overlapped with the "Unknown Regions". As more formal maps of the Star Wars galaxy were developed towards the late 1990's, however, major revisions were made to the Unknown Regions and galaxy as a whole. Essentially, there were fewer new opportunities to introduce entirely new interstellar races and empires for new storylines if virtually all of the galaxy had already been explored. In the original conception, the Galactic Republic/Empire controlled nearly all of the explorable galaxy, save for a few neutral pockets such as the Hutts, and the "Unknown Regions" beyond the Outer Rim were scattered fringe worlds. New galaxy-affecting political powers couldn't really originate from such meager areas, however, so new story revisions were introduced: the Galactic Republic and Empire did not actually explore and control 90-99% of the galaxy, but only around two thirds to three fourths. The explanation given was that most of the major stable hyperspace lanes leading out from the Core worlds led into the galactic "east", so colonization waves from the Core didn't actually radiate outwards in a perfect circle but were lopsided towards the east ("east" defined as if you put Coruscant at the "north" end of the Core Worlds). The result of this was that instead of the Unknown Regions just being the meager outer fringes of the galactic spiral, now large swaths of the galaxy remained to be explored in the galactic "west" - roughly around a fourth to a third of the galaxy. Thus new and powerful alien empires could be introduced from within the Unknown Regions, such as the Chiss. In time, "Unknown Regions" came to refer more to this unexplored quarter-slice of the galactic circle, while "Wild Space" tended to be used more for the scattered systems beyond the Outer Rim.
- The Western Reaches - When the explanation was developed that roughly the western fourth to third of the galaxy was unexplored and part of the "Unknown Regions", this introduced a discrepancy in terminology. The colonization waves radiating outward from the Core were lopsided towards the east (and also north and south) of the galactic spiral, and only stopped when they reached the physical fringe of the galaxy: the Outer Rim. Thus the "Outer Rim" is both the physical edge of the galaxy, as well as the "outer rim" of the colonization waves. In contrast, the outermost colonization waves to the west, the borderland frontier worlds facing the Unknown Regions, might be the edge/rim of explored space, but they are not physically on the "outer rim" of the galactic spiral. To this end, the new term was developed to refer to these borderlands as "The Western Reaches" (also known as the Western Fringes, the Western Marches, etc.) Politically and economically they are basically frontier worlds just like the "Outer Rim" proper (or might be thought of as a sub-region of the Outer Rim), but they are also close enough to the Unknown Regions to be threatened by powerful alien empires that might exist there. The starship graveyard planet Jakku, introduced as a major location in the Sequel Trilogy, is located in the Western Reaches - in fact, Han Solo even mentions the term "Western Reaches" in dialogue in the movie, establishing it as also part of the new Canon continuity.
In the original Expanded Universe, after Endor the surviving Imperials were pushed back to a handful of sectors at the north end of the Outer Rim, bordering the Unknown Regions, where they became entrenched as the "Imperial Remnant". Maps produced for the new Canon continuity also place the First Order (a spiritual successor of sorts to the old EU's Imperial Remnant), in this same general location.
Pablo Hidalgo, one of the producers on Star Wars Rebels, released the first new Canon continuity map in the Force Awakens Visual Dictionary. Generally, many planets which were introduced in the movies themselves are located where they used to be in the old Expanded Universe - the map is just simplified to cut out thirty years' worth of other EU-exclusive planets. For example, Tatooine is still located in the galactic "southeast", as it was in the Legends continuity; Geonosis and Naboo are also located nearby (the Prequel trilogy established that they were located relatively close to each other). Hoth, Bespin, and Endor are located to the galactic south. Hutt space is in the galactic east, in the Mid Rim along the border with the Outer Rim. Kessel is located due east from that in the Outer Rim. Mon Cal is located in the galactic northeast in the Outer Rim; Kashyyyk and Mandalore are also in the northeast, though closer to the interior than the Outer Rim.
The exact galactic location of the Lothal system within the Outer Rim has not yet been established. The fact that the Ghost crew are able to make reasonably short trips to Kessel and Kashyyyk (as well as Mandalore's colonies such as Concord Dawn) may somewhat imply that it is in the galactic northeast - not near Tatooine in the southeast, and not in the northwest which will later be controlled by the imperial remnants that reorganize into the First Order; however, further confirmation is needed from Hidalgo. In Season 3, the Ghost crew also makes reasonably short trips to Dathomir, though in Season 2 they also made a more prolonged journey to Geonosis (which is right near Tatooine in the galactic southeast).