To sum up DotA 2 it is best described as an Action RTS (real time strategy), if your not sure what an RTS is, the starcraft and warcraft series are good examples. Games such as Age of empires, red alert or any of those are also considered RTS typed genre games. Originally DotA stands for Defense of the ancients which was a type of mod for warcraft 3 in its days which was a specific type of mod that made 30-60minute length games where you and somewhat of an army, who in most cases would be friends, would battle against an enemy force which could be computer generated opponents or real life peoples (which was most common in LAN based touraments back in the days). During the game you would earn gold for kills and could use this gold to earn items or consumables which could be upgraded over time. The large discrepancy of what genre this is considered is because that over the game length you would essentially hit a max level and near that point the game would finish. If you were to play again, you would restart a game at level 1 to battle a whole new "army". Essentially, this shows RPG like game mechanics, but since you are hitting max level an infinite amount of times, it is best described as an Action RTS. Not long after the DotA warcraft mod was released, it became widely populated by people all over the world and soonafter games like League of Legends were created which are the same mechanics as the original DotA but updated and maintained a lot like other games (Starcraft, world of warcraft, ect).
So, DotA 2 is coming back but with an all new release and a brand new plateform which is attempting to redifine its series.
Incase your wondering, its being made by Valve. Yes, the creators of portal 1 + 2.
Yes, remember portal 1? it wasn't considered a sole game, it was included in the orange box collection which came with a couple of games (portal 2, team fortress, ect) so it was considered a lot like an expansion or mod since it was a never before seen genre. DotA and portal 1 are similar stories because of this, and if you see how successful portal 2 was on release and the scores its recieve, im thinking the DotA predecessor is going to be incredible!
ME and a lot of players from League of legends, and the original DotA MOBA are patiently waiting for this game and will be following it all the way to its release!
More info can be found here - dota2/
Here is an aritcle on DotA 2 - Game informer article