Warhammmer 40,000 is a tabletop strategic game, which means that people collect little plastic and metal miniatures, paint them and use them for a game, just like a 3-in-1 hobby. These battles are played by at least 2 persons, together with a bunch of dice, yardsticks, rulebooks and the like. Each miniature has its own profile. The players organise their armies into HQ-units, troops, elites, fast attack, and heavy support and fight for about a couple of hours. Kinda like online gaming, but then in real.
The story goes about that thirty-eight thousand years in the future, the mighty Imperium of Man has spread across the galaxy, to discover that the galaxy is a hell that would make Hieronymous Bosch shit himself in terror, and that it has a hell. From without, the Imperium is assailed by alien monsters from the depths of space, nightmare death-machines and soulless daemons; from within, treachery, heresy, mindless incompetence and the festering taint of Chaos threaten to tear it apart.
Warhammer 40,000 is not a happy place. Rather than just being Darker And Edgier, it paints itself black and hurls itself over the edge. The basic premise of 40k, as far as it can be summed up, is that of an eternal, impossibly vast conflict between a number of absurdly powerful aliens and the like. The basic weapon of a human Space Marine is a fully automatic armour-piercing grenade launcher.
The 40k universe is a spectacularly brutal playground of tropes and horrible things taken to their absolute extreme. The Imperium is protected by two different armies: the superhuman Space Marines and the massive armies of the Imperial Guard. Trillions of soldiers in its regular armies take disregard for human life further than most people could believe possible. A futuristic space Inquisition ruthlessly hunts down anyone with even a hint of the taint of the heretic, the mutant, or the alien. There's a Bug Swarm trying to eat everything in the galaxy, a light-years wide hole in reality through which countless daemons and corrupted daemon-powered super-soldiers periodically attempt to destroy the universe. Everywhere you go, there's the dumbass greenskinned Ork species that's infesting every corner of the galaxy and cheerfully trying to kill everything else in the galaxy because it's literally hard-wired into their genetic code.
And that is just the tip of the iceberg. Warhammer 40k is generally played by people of 12-30 years old, but those that have Warhammer as a hobby are mostly described by others as geeks, nerds or retards. That’s all prejudging. Most people don’t even know where they are talking about. The only thing that sucks is that the stores that sell Warhammer (Games Workshop mainly) are raping their customers because they ask £20, €30 or $40 for about fifteen plastic warriors or one tank.
The one thing you must know about Warhammer 40,000
Thou shallst always remember these wise words: Space Marines are t3h uberz! Hurr!