Warhammer 40000 (also known as Warhammer 40k) is a future universe, created by Games Workshop in 1987. Originally created as a tabletop minature wargame to accompany Warhammer Fantasy Battle, before long expansion packs became available, and other tabletop games were created that were set in the Warhammer 40000 universe.
The Warhammer universe is set 38000 years in the future where Mankind is led by the all powerful "Emperor", keeping the hordes of chaos at bay with his supreme psychic powers. Born a normal man who possessed an incredible intellect and enormous physical strength, he was also the most powerful psyker (psychic warrior) that mankind had ever seen.
In an effort to unite humanity and defeat the powers of chaos, the Emperor created the Space Marines, elite human warriors with superhuman abilities created through genetic therapy. The Emperor also taught humanity how to travel to the stars through the use of the Warp.
When his own son Horus became a traitor, the Emperor fought him in single combat, mortally wounded the Emperor defeated Horus and was put on a life support device called the Golden Throne. There he has sat for ten thousand years with his shattered body, intact only in spirit, with his incredible psychic powers protecting mankind.
Along with mankind, the universe is populated by Orks, Eldar and a whole host of other alien races, along with the armies of Chaos. The Warhammer 40000 novels are written by a number of very talented authors and published by Games Workshops own publishing house, the Black Library, and the primary focus is on the lives of the Space Marines and the Imperium.
Any sufficiently advanced technology is indistinguishable from magic