Space Hulk is a two player game in which one player controls a strike team of Blood Angels Terminators tasked with cleansing a massive space hulk (which is a roaming conglomeration of ancient spaceships that kinda of merged together).
The other player opposes the Blood Angels by controlling the genestealers. The board is layed out with various tiles with squares on them to represent the layout of rooms/corridors in the space hulk. The corridors are often only wide enough to admit the terminators in single file, making the order in which your termis enter a corridor very important.
Genestealer locations are represented by blips that the genestealer player can move around, until such a time as the Blood Angels player gains line of sight to a blip, at which point the blip is turned over to reveal how many genestealers that blip represents, and the stealers are placed on the board at that location.
The game is played in a series of missions, each mission having a specific objective for the blood angels player (the first mission for example, is to get the heavy flamer armed terminator to a certain room so he can destroy its contents, while another mission is to escort a robot around).
The missions each have different arrays of forces for both teams, and various special rules to make them interesting. The rules are reasonably simple and easy to play, and are something 40k or fantasy players will find pretty familiar.
