Object of the game
Missions
Each mission is different, but usually the emphasis is on the Marines having a task to complete before they are wiped out, and the Stealers having to prevent this. For instance, the Marines might need to last a certain number of turns; or perhaps they have to retrieve a game object from part of the board.
Forces
The Marines have 5 or 10 men on their side, including some special types (Sergeants and Flamer Marines) with different or better weapons and attributes. The Stealers may have a force of blips (tokens that represent the presence of 1-3 actual Stealers) to begin with, and in addition will receive at least one extra blip per turn as reinforcements. Blips are converted into Stealers at will by the Stealer player (or when a Marine catches sight of one).
Secrecy
At any time until it is converted, the value of a blip is opaque to the Marine player. Likewise the number of CPs (Command Points) which the Marine player has is not known to the Stealer player (this is an advantage to the Marines because they use their CPs to interrupt Stealers as they move).