Ok dudes, if you don't know Dota 2 is a video game. Its genre is hard to explain, some try to say MOBA(Multiplayer Online Battle Arena) but the most basic one would be Action real time strategy. But it basically derived from a Warcraft 3 custom map called Defense of the Ancient. It got pretty popular there, and just blew up, and ended up having actual development teams which is rare for a custom map.
Valve hired those dudes and started development on Dota 2, which is currently in beta on steam. The games hard to explain but this guy does it well
http://www.youtube.com/watch?v=Aqb54kHMvDQ
Its a very team oriented game and is fun because of the variety of heroes that there are.