It's pretty much Defense Of The Ancients, for any of you guys who know of that. If not, it's kind of like an RTS/RPG kind of game, where you control a champion. The thing is, you're part of a team of 5 (4 other champions)....and since I suck and describing things, here's what wikipedia has to say
Players are formed into 2 even teams of Champions, 3v3 or 5v5. Each team starts at opposing sides of a map in an area called the "fountain", near what is called a "Nexus". A match is won when either the opposing team's Nexus is destroyed or the other team surrenders. To destroy a Nexus, each team must work through a series of towers called "Turrets". Turrets are often placed along a path to each base referred to as a "Lane". Along the way, each player gains levels from killing the opposing team's champions and "Minions" (small NPCs that constantly spawn and attack the other team) and defeating neutral monsters (some of which grant buffs known as crests upon death). Completing objectives rewards players with gold which is used to purchase items. In League of Legends, each player starts at level 1 at the beginning of the match and can obtain the maximum of level 18 with their champion, leveling 4 different champion-specific abilities.