Well the standard intoduction to Warhammer things is:
It is the forty-first millenium.
For more than a hundred centuries, the Emperor has sat immobile on the Golden Throne of Earth. He is the master of mankind by the will of the gods and master of a million worlds by the strength of his inexhaustable armies. He is a rotting carcass writhing invisibly with power from the Dark Age of technology. He is the Carrion Lord of the Imperium for whom a thousand souls are acrificed every day, so he may never truly die.
Yet in his deathless state, the Emperor continues his eternal vigilance. Mighty battlefleets cross the Daemon-infested miasma of the Warp, the only route between distant stars, their way lit by the Astronomicon, the psychic manifestation of the Emperor's will. VAst armies give battle in his name on uncounted worlds. Greatest amongst his soldiers are the Adeptus Astartes, the Space MArines, bio-engineered super soldiers. Their comrades in arms are legion: the Imperial Guard and countless planetary defence forces, the ever-vigilant Inquisition and the Tech-Priests of the Adeptus Mechanicus, to name but a few. But for all their multitudes, they are barely enough to hold off the ver present threat from aliens, heretics, mutants - and worse.
To be a man in such a time is to be one amongst untold billions. It is to live in the cruellest and most bloody regime imaginable. These are the tales of those times. Forget the power of technology and science, forso much has been forgotten, never to be relearned. Forget the promise of truth and understanding, for in the grim, dark future there is only war. There is no peace amongst the stars, only an eternity of carnage and slaughter, and the laughter of thirsting gods.
Warhammer is basically a huge melee of various cliches fighting it out. It used to be more fun and goofy, if that's the right word, but they turn it more towards grimdark with every revision. In this, you're an imperial guardsman, a disposable soldier that defeats the huge, monstrous enemies with a lasrifle (a pew pew lasergun) and sheer numbers.