you could fight on either the side of the United States Colonial Marines or on the side of the Aliens.
The Colonial Marine faction had the Medic, Scout, Soldier, and Heavy Weapons classes. More advanced weaponry and increased hit points became available as a character progressed in rank. Marines' equipment included the motion detector from the movie Aliens, which was their sole method to locate Aliens, but would not locate Aliens which were not moving. In later versions of the game, the Marines could also drop proximity mines.
The Hive Alien faction had the Face Hugger, Drone, Queen, and Empress classes. Their capabilities included stealth, leaping, and radar of the entire map, representing the aliens' greater awareness and hive-like mentality. They attacked only at close range using tail strikes and with claws. Each map allowed one alien player to fill the role as Queen or Empress, and that player could freely swap from drone to drone. Players gained points for killing Marines and lost them by dying, except when playing as a face hugger, which loses no points for dying. Players had to score 1000 points before they could play as Queen and 5000 before playing as Empress.
Scenarios were played in one of six different types of terrain sets, with upwards of twenty to thirty players per side during each instance.