Numion said:
At least in the older books the emperor is not only the leader of mankind; he's essential in navigating the space. His soul shines through the warpspace like a beacon (the Astronomicon), allowing mankind to travel faster than light. Without the beacon ships would get helplessly lost, and mankind would be lost.
It's not without a price, though. Hundreds of psychic humans have to be sacrificed to the emperor daily to keep his failing psyche working as the beacon.
Also in the slaves to darkness books it was told that all humans, except inquisitors, were killed if they even knew about the existence of demons.
Ya gotta love that conflicting fluff.
There is a whole segment of Space Marines, the Grey Knights that are pskyic and fight demons, as does any Space Marine force thats around the Eye of Terror.
So tibits I can think of.
Hyperspace is the "Warp" is a psykic dimenson where the evil gods dwell.
One of the first evil gods was given birth by the race of Eldar (Space Elves) via the their pleasure and excess. The event obliterated their homeplanet and now they move around in huge ships called "craft worlds." Any reall information on those that fell to Slannesh, the god of Pleasure, called Dark Eldar has only come light in the latest edition of the game.
The Space Orks are like Orks with Cockney accents and guns. They actualy are unkillable,but not unstoppable since they regenerate over time. Most of their beasts of burden are actualy mobile fungus. In fact, I think they are fungus.
Squats are Space Dwarves, which was an offshoot of the human race that landed on heavy G planets. They had steampunk, underground artilary and Harley Davidson trikes. They have been phased out.
On the flip side, the Tryannids where just a race that used organic technology (their radios have mouths) But have now become a monstrosity of Starship Trooper porportions. Their popularity was enhanced by adding the Alien rip-off genestealers to their army force. The same marketing tactic didn't save the zoat modeles though, and the stunt even made some people swear off 40K. (me)
The Space Marines come in all shapes and sizes and have slight mutations to make them more wolf or vampire like for you taste.
I don't know much about the new races, the Necron, Tau or Dark Eldar.