Actually it's very different. I'll explain, for those who don't know:
Warhammer 40,000 is a futuristic tabletop wargame, which revolves around hobby modeling. Basically, you command several squads that you purchased based on a pre-determined points scale (my army, for example, is a thousand points). Though you put the individual troops together using very detailed plastic and metal model kits, they are very customizable, and depending what you equip each model with, they can do different things.
Different hero characters and vehicles are also available, as well as many upgrades fro all units. The game has rules detailing shooting (including missiles, mine fields, super-heavy weapons, etc.), close combat (including stuff like chainsaw swords), and morale (if a squad gets its ass kicked it will actually run away, sometimes retreating out of the game entirely).
40k is home to the most adrenal, brutality-fueled sci-fi backstory I've ever seen. Basically, it's the 41st millenium and humankind has changed severely in the time between now and then; we're religious zealots hell-bent on crusading and eliminating those would prevent our empire from prospering, giving our lives in service to the crusades of the immortal Emperor, a being so revered that the entire earth has been converted into a technological throne to hold his debilitated body.
40k is gothic-inspired, but it has a great slew of races and conflicts raging, and some of the best sci-fi storylines ever, far as I'm concerned. There are the Necrons, an ancient race of silent, stalking skeleton-machines who can nearly never die; the Space Marines, massively genetically augmented supersoldiers of the Imperium of Man; the Tyranids, which are basically a much more vicious and grim version of the Zerg; the Eldar, a race of graceful humanoids who once ruled the stars and now are nearer to extinction then ruling the galaxy; the Chaos Marines, who are space marines tainted by demonic energies from a subconscious space of malice-filled thought called the Warp; and of course, the Orks--a race bred solely for battle, complete with comically poor technology and aussie/brogue accents.
Spawned from this hobby/tabletop game are the PC Game series Dawn of War, so if you ever are interested in seeing the game firsthand you can download demos of the PC game and its expansions from fileplanet.
Warhammer 40k (and its fantasy counterpart) are actually the key inspiration behind legendary video game franchises Warcraft and Starcraft, respectively. It's almost too obvious when you look at the two--and that's coming from a Warcraft fan, heh.
