Well here it goes. First there's the evil aliens that threaten the galaxy. They plan on assimilating all life that isn't their kind. They have no "technology". Just living organisms with different tasks. Some are even buildings! Second: You have a shiny, deeply religious, technologically advanced alien race with all sorts of brightly colored buildings, vehicles, and weapons. Weapons are energy based and everything seems to glow. You also have a small group that sympathizes with the humans. Lastly: You have us. We're caught in the middle. We want to destroy the evil organic aliens while not getting killed by the religious fanatics.
Both can be used to describe Starcraft AND Halo. Whoops! ^_^