Potentially the hottest game to be released in 2009, Call of Duty: Modern Warfare 2 was on everyones buy list selling approximately 4.7 million copies worldwide in the first 24 hours of release and as of January 18, 2010 taken over $1 billion in sales.
The single player mode of CoD MW2 will have you playing 5 different characters from the first-person perspective focusing on a group called the Ultranationalists who seize control of Russia and eventually invade the US. You will start off in Afghanistan but also find yourself in the US and other locales.
CoD MW2 multiplayer mode offers several game modes including Capture The Flag, Demolition, Domination, Free-For-All, Search and Destroy, and Team Deathmatch. You'll find yourself earning experience points and unlocking rewards as you play.
See Wikipedia for more information and Kotaku for a review.