There is not plot or background. Anyone with a green triangle over his head is a friend. Those with Red triangles must be killed. However, it is the execution, so to speak, of these hard and fast rules that are truly engaging. Perhaps most interestingly, Tribes allows players to take on very specific roles.
Individuals can choose roles that suit their skills.
Tribes breaks this mold. The choice is yours. Tribes 2, the newest release by Dynamix takes these roles even further. These packs are perhaps the defining aspect of a player's role in the game. With a repair pack, you can move around your base repairing turret emplacements and your fellow defenders.