Developer website: tremulous.net
Player advancement is different depending on which team you are on. As a human, players are rewarded with credits for each alien kill. These credits may be used to purchase new weapons and upgrades from the "Armoury". The alien team advances quite differently. Upon killing a human foe, the alien is able to evolve into a new class. The more kills gained the more powerful the classes available.
The overall objective behind Tremulous is to eliminate the opposing team. This is achieved by not only killing the opposing players but also removing their ability to respawn by destroying their spawn structures.