The Dota 2 beta is now over and the game has now been officially launched. Dota 2 is free to play and can now be downloaded on Steam. For more information, check out the official "The Beta Is Over" page.
"Dota 2 consists of session-based online multiplayer matches in which two teams of five players attempt to destroy their opponents' fortified strongholds. Each player controls a "hero" character and, over the course of an individual game, focuses on improving their hero's abilities, acquiring items, and fighting against the other team"