In the WH40K setting, the Imperium of Man has expanded into much of the Milky Way galaxy, and is interacting with a number of sentient extraterrestrials. These include the Tau, a young and technologically-sophisticated civilization of aliens that work for the "greater good" of their empire and its inhabitants; the Necrons, soulless, living-metal constructs tricked into slavery by star-devouring creatures called the C'tan; the Eldar, an ancient and arrogant race whose psykers can predict the future, and their tainted and forsaken brethren, the Dark Eldar, who must consume the souls of others to make up for the loss of their own; the Tyranids, an all-consuming, all-organic, bio-engineered, extragalactic swarm; and the Orks, whose boisterous personality, tactics and biology make them the comic relief of the series. Each of these races have playable figurines. Other playable armies include the Witch Hunters, Daemonhunters, Sisters of Battle and the Imperial Guard, organizations within the Imperium; the Kroot, first introduced as a member of the Tau consortium; the fallen Dark Eldar; and the capricious Daemons of Chaos. The four gods of Chaos – Khorne, Tzeentch, Nurgle and Slaanesh – are embodiments of vice who attempt to corrupt members of all races to serve their ends. These four "Ruinous Powers" live in a parallel dimension called "the Warp" or "the Immaterium," which is utilized for interstellar travel and can be channeled by psykers. Psykers make up the bulk of the Imperium's information infrastructure, handling interstellar navigation and communication, but are always at risk of being perverted by the Warp or being possessed by daemons. The Ruinous Powers also make their presence known through a rip in the galaxy at a nebula called the "Eye of Terror," where the Warp has broken through into normal space. From here Abaddon the Despoiler, Warmaster of the servants of Chaos, launches "Black Crusades" to (attempt to) conquer the galaxy in service of his masters.
While the bulk of humanity's military power is found in the Imperial Guard, the Space Marines – giant, genetically-enhanced super-soldiers with world-destroying firepower and unswerving, fanatical loyalty to the Emperor of Mankind – are the most famous. The Emperor created the Space Marines, and their leaders the Primarchs, for use in "The Great Crusade," a two-century effort to re-unite the farflung colonies of humanity following a "dark age" known as the "Age of Strife". Unfortunately, some of the Primarchs gave their devotion to the Ruinous Powers, becoming the Chaos Space Marines and bringing war to Holy Terra during a conflict known as the Horus Heresy, after the Primarch who led the attack. This war left the Emperor crippled and dying, and he has since been entombed in the Golden Throne, a comprehensive life-support apparatus that sustains his body as he uses his vast psychic powers to combat the gods of Chaos and help smooth interstellar travel.
For a complete list of playable factions, see the factions page.