Okay, so, the basics of the 40K universe.
It is the distant future. Humanity has spread to the farthest corners of the galaxy, but has fallen from the lofty heights attained during the Dark Age of Technology. These days, technology is the sole province of the priesthood known as the Adeptus Mechanicus who regard it with reverence and take a dim view of innovation. Humanity exists on uncounted worlds, in states ranging from savagery to feudalism to enlightenment (for a given value of enlightenment) and all points between. Only their mutual allegience to the Imperium and worship of the Emperor unites many of these worlds, and beyond Imperial space even these common factors fall away.
The Warp - A realm of pure energy given form by the minds of sentient races, the warp is a source of limitless power and the medium by which starships traverse the vast gulfs between the stars. It is not a friendly place. Countless entities, large and small, inhabit the warp, all hungering for the souls of mortals. Most terrible of all are the four Chaos Gods, who endlessly scheme to remake the galaxy to their own whims. In some areas, the warp spills over into real space, resulting in realms where men and worlds become twisted, and the laws of physics collapse.
Psykers - Psykers are individuals able to channel the power of the warp, allowing them to bend reality to their will. Such power comes at a price, however. The mind of a psyker is a shining beacon to the daemons of the warp, and only the strongest willed, or those protected by the Emperor's might can hope to survive long without becoming the vessel of damnation. Nevertheless, psykers are vital to the survival of the Imperium. Astropaths allow interstellar communication, while navigators can navigate the shifting currents of the warp in something resembling safety. Psykers are therefore a necessary but reviled group.
The Major Players:
The Imperium - Aguably the largest, most united power in the galaxy, the Imperium of Man stretches across the galaxy, though it by no means encompasses every star. With the Emperor confined to his golden throne these past 10,000 years, it falls to others to rule in his name. Imperial citizens are subjected to the whims of vast and faceless organisations, and those of all-too-immediate planetary governors, nobles and military commanders. Each world is expected to contribute a tithe towards their common defence, and the High Lords of Terra do not tolerate secession.
The Orks - Live to fight, fight to live. Such is the motto of the ork. A race of hulking green-skinned savages found thoughout the galaxy, the average ork has little room in its head for anything beyond violence. And yet, they somehow maintain advanced technology (for a given definition of 'advanced'). Each ork has a multitude of complex information encoded into its very DNA by a vanished creator race, sufficient to propel them from the stone age to the stars in a frighteningly short interval. Orks are always at war with somebody, and failing other enemies, they will happily fight amongst themselves.
The Traitor Legions - 10,000 years ago, the Emperor's favoured son rose up against him, plunging the fledgling Imperium into civil war. Today, the survivors of that war brood upon their defeat and plot revenge. Granted immortality at the cost of their souls, the Chaos Space Marines are driven by their hatred to wage eternal war against the Imperium. Insane and depraved from millennia of worshipping dark gods, the traitor legions may make use of mortal pawns or summon daemons to do their bidding.
The Eldar - Once the rulers of the stars, the Eldar are now a dying race, refugees from a terrible cataclysm known as the Fall. Possessed of mighty but alien intellects, the Eldar are famed for their intricate and incomprehensible scheming. Although few in number, they still possess all the fearsome technologies and witchcraft of their glory days, and can seldom be brought to battle save when they so choose.
The Tyranids - Invaders from beyond the galaxy, the tyranids have brought bioengineering to its ultimate extreme, with every organism in the hive fleets bred for a specific purpose. Where they pass, the tyranids leave entire worlds stripped to the bedrock in their wake, their biomass incorporated into the ever-moving fleets. Utterly implacable, the tyranids can be halted only at great cost, and even the world-destroying fires of Exterminatus are but a last, desperate gambit against them.