Inspired by DOTA (custom mod for warcraft3), where you pick one of the 70ish available heroes and team with 4 other people to compete against another team of 5. Gameplay is incredibly rich, it is a game that you can play for years without ever tiring of it because playing with and against other players makes the experience much more deep.
However I must warn you that the learning curve is pretty steep, you will be called a noob, you will get yelled at, it will take you a couple of weeks to get used to all the heroes, their abilities and the various items available. However, the guys at S2 have added a new game mode, which is supposed to be much easier, with a smoother learning curve - casual mode. The game becomes overall more forgiving of mistakes, for example there is no more gold penalty on death.
Definitely one of my favorite games on PC