http://www.natural-selection.orgIt's a mix of RTS and FPS. The commander of teh Marines plays from a top-down view, and ordrers his marines, while controlling upgrades and purchasing weapons. The aliens are under less control, though this often means fewer coordinated attacks. The objectives of both teams are to collect resources and destroy the other team. Resources are needed for upgrades like gun upgrades for the marines or evolution upgrades for the aliens. Destroying the other team's Hive, if you're a marine, or Command Console, if you're an alien, will win your team the game.
There's much more, but that's just a quick intro.