A few months ago, Valve trademarked "Dota," a seemingly meaningless phrase when written in all lower-case letters. When written like "DoTA," however, that phrase is a reference to "Defense of the Ancients," an extremely popular (and free) mod for "WarCraft 3" which blends tower defense with RPG-style character building.
Valve's partnership with Icefrog, one of the main developers for "Defense of the Ancients," made it clear that they were moving into the franchise's territory with an upcoming game. That game is "Dota 2."
GameInformer has all the details on "Dota 2," but their site is currently getting hammered, so I'll try to summarize the high points for you.
Same Gameplay, New Visuals
According to the article, the gameplay in "Dota 2" is "almost entirely untouched." All the heroes from "DoTA Allstars" will be appearing in "Dota 2" and most of the items, skills and upgrades work as fans will remember them.
The biggest change between "DoTA" and "Dota 2" will be on the graphical side of things. "Dota 2" will run on Valve's versatile Source engine, and new visual effects like cloth effects and and improved lightning will be added.
A lot of additional back-end work will be done, as well. Players will be able to chat using in-game servers rather than 3rd party programs, and bot matches will be accessible for training and when other players disconnect.
There's a bunch more to discover about "Dota 2," so be sure to head over to GameInformer and read all about it. The site should settle down in an hour or two, so be patient when attempting to load the page.
This reveal does bring up some interesting questions about who actually owns the rights to this content. Much of "DoTA" was created by community members, which begs the question whether Valve has the rights to sell this content as a separate game. Who really "owns" the content created for a free mod? I'm sure this is all stuff that Valve's lawyers looked at before moving forward on the "Dota 2" deal, but it'll be interesting to see if it ends up causing any strife.