From the publisher's website:

The game is set on a modular board made up of various board sections which represent corridors and rooms and which can be freely arranged and locked together like a jigsaw puzzle to represent the interior of a space hulk. One player controls the Space Marine Terminators, and the other player controls the Tyranid Genestealers. It is an asymmetric game. First, in that the two players have different forces to begin with. Second, in that the two players may have different objectives to fulfill during a "mission" (the in-game term for a particular scenario of the game). The Terminator player may have the objective of destroying a specified area of the board or a specified Genestealer piece or some other objective; and the Genestealer player may have the objective of destroying a specified Terminator piece or all of the Terminator pieces or some other objective. Third, in that the pieces of the two players move differently. The Terminator pieces move slowly; and the Genestealer pieces move quickly. Fourth, in that in regard to combat: the Terminator pieces excel in ranged combat, but they are weak in close combat; and the Genestealer pieces excel in close combat, but they cannot perform ranged combat at all. The pieces are moved by the players through a system of "action points", where each piece has a certain amount specified for it. A Terminator piece has few action points; and a Genestealer piece has many action points. The game is notable for its hidden play mechanics, from which it derives much of its playability and tension. On the one hand, the Terminator player has a variable number of "command points" available each turn which are only revealed to the Genestealer player after they are used up; and these command points may be used to move the Terminator pieces either during the Terminator player's own turn or during the Genestealer player's turn. (In the second edition, the extra points were not hidden from the Genestealer player.) On the other hand, the actual number of Genestealer pieces in play is hidden from the Terminator player, because the Genestealer pieces come into play as "blips" which can represent: 1-3 creatures (0-3 in the Deathwing expansion and 1-6 in the Genestealer expansion) in the 1st edition; 0-6 creatures in the 2nd edition; 1-3 creatures in the 3rd edition; and 1-3 creatures in the 4th edition. In the basic version of the game: playing as the Terminators can be engaging and tactically challenging, partly because the Terminator player is constrained by a time limit for his or her turn; while playing as the Genestealers can be very straightforward. To overcome this asymmetry, players are encouraged to play each mission of the game twice, swapping between playing with Terminators and playing with Genestealers. The fairly fast play time (around half an hour per mission), driven by the Terminator player's time limitation, makes this possible. On the one hand, the Deathwing expansion pack for the first edition included rules for playing the game as a one player game, that is playing the game as Termintors using different mechanics for moving the Genestealer pieces. On the other hand, the Genestealer expansion pack for the first edition included rules for human-genestealer hybrids, which could carry weapons and equipment, and for a Magus and for a Patriarch, adding more depth for the Genestealer player. However, these additions were not carried over in the second or the third or the fourth editions, although, the third and the fourth editions did include rules for a Broodlord.