5000 years ago, the ship Florence left Earth's solar system, bound for the Alpha Centauri system. It carried 500 brave colonists, living and working on the ship. They didn't make it, succumbing to internal struggles and war. So more ships were sent. At first they were generational ships, carrying a mass of humanity living and breeding within their walls, and humanity was slowly but surely expanding. But eventually the scientists of Earth developed stasis technology, and space travel could be done within a single biological lifetime. Human colonization exploded, with tens of thousands of worlds being explored, terraformed, and colonized.
The catch to this expansion was simple: For the most part, inter-planetary travel took hundreds of years, making it massively impractical. In essence, this left every human planet nigh-totally isolated, with minimal communication, trade, and travel. This led to various different societies, and a mish-mash of new cultures. Some planets which were colonized early devolved into tribal societies after the colonies fell apart, and some planets went from tribalism back up through the various stages of human society, some even independently reaching a modern state.
As a result, no planet is the same. For instance, the planet El Dorado was colonized by an American colony ship in AF 278, after leaving Earth in AF 2. The people of this ship, being mostly political exiles and criminals, nigh-immediately devolved into a tribalistic society. For the next 2000 years, they wandered the earth as nomads, worshipping half-remembered American ideals centered around a statue of the founding fathers left in the town square of the original colony. Then another colony ship, the Pax Alpha Centauri III (from the Alpha Centauran Empire, an empire encompassing three planets), landed and established a new colony in AF 2302. The colony had incredibly poor relations with the El Doradan tribes due to establishing near the site of the original colony, and fought numerous skirmishes that they managed to win easily with their superior technology. However, the tribals proved to be too numerous and the colony was overrun. The El Doradan tribals, seeing that they were not alone in the galaxy and that the old legends were true, united under a single striped banner, stored in a shrine deep under the earth since the original colony, and quickly modernized. They made their own invasion ship in AF 4582, and set a massive army to Alpha Centauri. They have yet to arrive.
Other planets have medieval societies, societies so advanced as to be almost non-human, massive stretches of irradiated wasteland, or steam powered tech.
This galactic anarchy went on for almost 4000 years before any sentient, high-tech aliens were encountered. Then a colony ship from the high-tech world of Welthincfanamlater was intercepted mid-transit by an unidentified alien vessel. The aliens boarded the ship, and upon accidentally activating the security systems lost a large portion of their boarding party. This incident led to the Gruplok invasion, which has been invading worlds on the edge of the galaxy for centuries. They've been held back by a union of 15 closely grouped high-tech worlds, who have managed to use recovered Gruplok tech to ease communication and transport.
Only 22 years later, in AF 3993, the planet of Middle Earth (originally settled by Lord of the Rings fans for it's geographical resemblance to middle earth, who used genetic engineering and sheer willfullness to recreate as much of the world of Middle Earth as possible) was invaded by a race of green-skinned aliens calling themselves Orks. These Orks [Think WH40K] highly value war and valor, and were incredibly confused to encounter some stunted and evil humans calling themselves Orcs. They enslaved the Orcs of Middle Earth, and quickly destroyed massive swathes of middle earth before being driven off the planet by an army of "gods" wielding immensely powerful tech. The Orks moved on to another planets, and pillaged that, and then they just kept moving from planet to planet destroying and burning everything they saw. Sometimes they were driven off, sometimes they scoured and enslaved the surface, turning that planet into yet another Orkish breeding ground. One notable example is the planet of Poland. (Not to be confused with Poland, Poland, Poland III, Polska, Poleland, or Polland.)
In AF 4088, the colony ship Hurt landed on a planet. This planet was notable for having signs of alien ruins, and the colonists on Hurt were mostly scientists and archaeologists with the goal of studying and exploiting any alien tech/ruins. Instead they awakened a massive army of insectoid robots, which had presumably wiped out the aliens that lived there before. These robots immediately wiped out the colonists, and reverse-engineered their ship to craft thousands of small invasion crafts, which they proceeded to seed the galaxy with. These craft landed on the surface of the planet, and immediately built a massive manufactory and vault for the robots, who would use this as a staging ground to purge the planet of all organic life. Once they've finished with a planet, they go into a dormant state. Occaisionally more invasion crafts are sent out from one robot planet or another. There are hundreds of planets where the existing governments are fighting back, or have succesfully driven the robots off the planet, and hundreds if not thousands more planets where the robots won and have become dormant, waiting for another biological lifeform to re-activate them.
The fourth, and so far final, species of sentient aliens were not encountered in war. They were actually encountered when a massively high-tech world built a beacon to send signals into space, hoping to contact aliens, in AF 4221. They succeeded in encountering the Mi-Go, and were shocked to see that they resided on Pluto, which they called Yuggoth. The Mi-Go had actually come to earth in the decade of BF -200 (AD 1930). They were driven off by a particularly excitable young man, and had retired to their caverns on Pluto for the next 5000 years, watching the progress of humanity. They decided that, given the several hostile alien races, it was time to introduce themselves back to the humans. They are notable for having almost no technology, instead having evolved to fly through space totally undetectable on their great wings. Their primary goal is the preservation of species, which they do by placing the still-living brains of members of other species into special vats and tucking them away into dark vaults below pluto, where they talk to them and record what they say. They are neutral to humanity, but individual Mi-Go frequently live on and perhaps even fight for human planets.
It is in this universe that you live in. It is the year AF 5000. You could be anyone, from any planet, doing anything. The galaxy is nigh-boundless, with exactly 297,391 populated planets, and each one interesting and full of adventure. Go get 'em.