They specialize in long range attacks but are vulnerable up close against aliens. The goal is to exterminate the other team. To do this, you must eliminate all the opposing players and all of their spawn spots, so they cannot respawn. When a side is totally eliminated, the other team wins. For humans, they spawn from teleporters.
Aliens spawn from eggs, which regrow after an alien emerges.
Each side has one class which can build new spawns, create defenses and possibly repair friendly units. This class is the engineer for the humans and the Breeder for the aliens. Bindings Binding keys during gameplay isn't easy. Besides having to worry about being fragged while you're binding them, it's a hassle. So many keys to bind, such little time to play that mod you're trying to bind for.