EDIT: Ninja'd.
Space Hulk is a tactical strategy game. "Space hulks" refer to any number of ancient derelict ships, asteroids or other assorted space junk that eventually merge together into one huge mass. These can range in size from a small moon to a large planet. Space hulks have a tendency to slip in and out of the Warp, and can materialize just about anywhere with no warning.
The Imperium often sends strike teams to investigate space hulks, as they can contain treasure like lost secrets from the Dark Age of Technology and/or threats in the form of pirates or xenos. More often then not they stumble across enemies, and the strike team has to fight their way through.
A typical Space Hulk game has one player commanding a small squad of Space Marine Terminators (elite veterans with powerful weapons) while the other player controls Genestealers (Tyranid life form meant to scout and infiltrate for the Hive Fleets. Extremely deadly in close quarters). The Space Marine player has to navigate the Terminators through the hulk to their objective, while the Genestealer player has to try and stop them. Because of the hulk's claustrophobic hallways and corridors, the Marine player has to utilize proper tactics and strategy to get his Terminators past the 'stealer hordes.
While primarily a board game, there have been a few Space Hulk video games in the past, namely the 1993
Space Hulk and 1995
Space Hulk: Vengeance of the Blood Angels.
Definitely a little rough around the edges, but they tried to capture the tactical aspect of things.
There have also been a number of Space Hulk mods for various games, including
Dawn of War: Winter Assault,
Dawn of War II and
Left 4 Dead 2.
This newest iteration looks promising. I'm definitely keeping an eye on it.