This game is an Alien vs Marines fps/rts game. Each team has a commander than views their respective team from a "birds-eye" view and places buildings/defenses and issues orders to their team. The players Fight it out on maps where aliens try to spread infestation which changes the texture of the map's as it spread and marines try to destroy the alien home bases. The actual gameplay is not so dis-similar to counter-strike except one team doesn't have guns(aliens) and have to use different abilities to kill the marines. Although I have to say spitting acid, or biting off a marines head is imo, more satisfying than shooting something with a gun
There are a bunch of different classes of player, such as support/healer, Heavy Gunner, Flamethrower/grenade launcher, Giant Rhino-like Alien that head buts and stomps on marines etc etc.
It's currently in beta-stage, but will be released in maybe 6-10 months so If it interests you I would recommend it! It's produced by an indie studio so your money doesn't go to some big box company. The Beta is already lots of fun though and U can get a full version of the game and beta access for like 30$ I think it is.(although I think they are gonna have a 2 for 1 deal coming for christmas so if this interests you, you might want to wait, 2 copies of the game for the price of 1)
Also for you people who don't have fancy new computers the Requirements for the game are supposed to be very low when it is released, similar to CS:Source.
(Australia's own NS2HD has a bunch of videos about the game)http://www.naturalselection2.com/ http://www.unknownworlds.com/ns2/