DOTA 2 or Defense of the Ancients 2 is a game that has been long anticipated. It's predecessor was a warcraft 3 mod for the expansion pack : Warcraft 3 The Frozen Throne. Both of these games were made by Icefrog, a subdivision of the company we all know today as Blizzard. Shortly after the release of the dota mod, the team involved went their separate ways and worked on their own titles. Many similar games have sprouted from the same idea. Games such as League of Legends, Heroes of Newerth, Realm of the Champions, Smite and more are all inspired by the original ; DOTA. There is on ongoing debate about which game ripped of who, but in the end they all used similar ideas and created very similar heroes and gameplay.
DOTA2 - Developped by Valve. Is a free to play game on Steam, it's only revenue is through microstransactions in-game. Right now the game is still in closed beta, but a key can be obtained by applying through the steam store page for DOTA2.
The game is from the MOBA genre (Massive Online Battle Arena) This genre has the same idea and objective in any of it's games. Simply fight your way through waves of creeps that will attempt to make theit way towards the base. Destroy the heavy hitting towers along the way whilst worrying about the enemy team. Destroy the enemy base. Of course there is the item shop where you spend your hard earned gold on items to make your hero a beast on the battlefield. Simply, more gold = More success.
Mastering the heroes is a key element in winning, there are 3 main types : Strength, Agility and Intelligence. But within these catogories there are over 90 heroes all with unique abilities that can turn the tides of battle.
If you want to learn more you can read an in-depth review or watch a video review online.