I’ll try to be short, it’s only an introduction. Every race will have its own, big post.
THE IMPERIUM: Once upon a time, there was the Emperor, the mightiest man ever, who united mankind and lead it to conquer a great slice of galaxy. Too bad his favourite son Horus, tainted by the Ruinous Powers (or Chaos Gods, see below), fucked everything up with his betrayal and almost killed him. Since then he is in a 10k years coma within the Golden Throne, guiding mankind with his psychic might while his body stands on the brink of death. Now the Imperium is a xenophobic, warlike and close-minded gargantuan empire, but in times like these, you can’t afford to be nice. Everything non-human is xenos to be exterminated, and everyone speaking against the Imperium is a heretic to be purged. Thus, the Imperium is costantly at war with someone (literally), deploying all its military might, being the super-badass Space Marines or the disposable and uncountable balls-of-steel Imperial Guard. It’s a matter of survival in this unforgiving universe.
CHAOS: The Warp, an infinite dimension of psychic power, is home to the Chaos gods, and therefore a place of absolute madness, perversion and malice. Said gods, shaped and fueled by nightmares, negative emotions and perversions of the living, costantly plot and act to make the galaxy look like their own realm by making their worshippers kill without reason, desecrate anything and indulge in any kind of wicked shit. Those men (mainly Space Marines) who followed Horus’ betrayal in the sign of Chaos now make up the Traitor Legions, and live in the Eye Of Terror, a kind of enormous portal to the Warp. Those guys are the worse enemies of the Imperium and boast some strong powers received by Chaos itself, at the price of deformities, body alterations, plagues, diseases and stuff you could expect to receive along evil powers. And lots of spikes.
ELDAR: One of the most ancient races in the galaxy (think space elves, ok?), had a big empire too, back in the day. But their being very powerful in the psychic field and advanced in technology made them lazy at first, and degenerate later. Not having work or war to spend the time with, they started to indulge first in philosophy and otium, then in perverted parties and feasts, becoming more and more hedonistic. Their perversion was so great it slowly fueled a nascent entity in the Warp, until with a colossal bellow, the fabric of reality was torn apart and a new Chaos God, Slaneesh, was born. He/She consumed all the Eldar souls he/she could find, almost wiping them out of the universe. Being created by their desires, still Slaanesh claims every Eldar soul when one of them dies (unless it’s stored in a Soul Stone). Some survivors became much wiser, but some others couldn’t give up the parties (Dark Eldar).
DARK ELDAR: They are the evil counterpart of the surviving Eldar. They found shelter in the Webway (another dimension used for faster-than-light travel) in the city of Comorragh, but still Slaanesh claims their souls and asses, slowly draining them into the Warp. While becoming more and more evil and perverted, they found out the curse could be avoided making others suffer in their place. Basically they keep themselves alive by feasting on misery and suffering of other intelligent beings, more suffering the better. To this end, they became pirates and slavers all over the universe, capturing everyone they could find to bring them back to Comorragh as slaves and have some atrocious fun.
ORKS: Imagine Orcs, in space. The most warlike of all races, and almost the most numerous (along with humans, second only to Tyranids). Despite most of them being stupid as a brick, they are proficient in all kinds of technology because of their genetic heritage. Their success comes from their reproduction system, which is similar to a fungus (every Ork leaves spores around and, given time, a fully formated Ork will be generated. Therefore, their numbers are always very big) and their redundancy of vital organs, which makes them HARD to kill. They can make vehicles and weapons from scrap, and this stuff works because THEY BELIEVE IT. They have no psychic presence, but rather a collective mind-field. They think red vehicles go faster, SO THEY DO. Sometimes a charismatic leader unites a good number of tribes to raid/conquer/destroy some part of universe, and a WAAAGH! is born. Fortunately, a WAAAGH! never brought all Orks under one banner, or it would be the end of all other races.
NECRONS: They are a race of sentient, yet emotionless, robots. Once properly living, they gave up flesh and soul in order to achieve immortality and defeat their ancient enemies, the Old Ones. After the War in Heaven, their pride made them turn against the same divine entities, the C’tan, they used to worship and which gave them their immortal bodies. They won, but the price was so high they understood they could not face the rising Eldar empire, former allies to the Old Ones and their worst enemy, so they went on a 60 million years long stasis in hundreds of tomb worlds. The Eldar empire fell to its own hand, and now the Necrons are slowly waking, world after world, ready to start their conquest anew. They are very much inspired by ancient egyptian culture and its emphasis on death, and in combat their warriors can rise again after being slain, while also being very hard to kill in the first place. Remember T-800 from Terminator? The Immortals are pretty much that tough.
TAU: The Tau are the youngest race, sporting an amazing technology rush, going from the discover of wheel to space travel in less than 5.000 years. In this universe, they are some kind of “good guys”, meaning they try to use diplomacy and life in their domain is quite good. They always ask their would-be enemies (in the case of intelligent beings like humans or Eldar) if they want to join the “greater good”, their chief philosophy. Of course this is Warhammer 40k, so if they don’t, the Tau deploy their advanced military might, go guns blazing and, if they win, they might use re-educational camps much like soviet Russia did to have people change their minds. This, and their Greater Good concept, earned them the humorous title of Space Commies.
TYRANIDS: They come from the cold void outside the galaxy. They only care about one thing: EAT. The most vicious of all races, they are the stuff of nightmares. They look like insectoid life forms, costantly evolving and becoming more and more lethal. When they reach a planet, they consume everything. Only a ball of barren rock remains when the swarm departs. They travel the galaxy in giant Hive Fleets, counting each billions of them, along with Hive Ships, massive living ships which use biomass to generate more Tyranids. Fortunately, they are incapable of faster-than-light travel (except for short routes when approaching a planet), or else they would have already consumed all galaxy. The only races who can really survive them are the Orks, thanks to their fast reproduction and vast numbers, and the Necrons, since living metal is not food for them.
Thanks for reading! Your feedback is welcome, as well as preferences about next Warhammer post!