Warhead is the expansion pack that was released yesterday. It adds a new chapter to the game that actually provides more story and information then the original Single-Player game did. Plus it adds in performance enhancements and tweaks for system optimization. From a MP perspective it adds some new things as well. So far based on GameRankings.com scores, it's being very well recieved and is a must buy if you were a fan of the original game.

*cheers!*