I think it's best described as King of the Hill mode. Except there's 5 separate hills spread across a map.
The way it works is that each side's Nexus has a set amount of hit points. By controlling more capture points than the opponent, the opponent's Nexus begins to lose hit points. The team whose Nexus hits 0 first loses.
They've also made quite a few changes from regular DotA, as well. Namely, far less creeps (but they're a lot more powerful), turrets being an actual threat even in endgame, the passive gold generation is dramatically increased, and everyone takes more damage/heals for less.
Their goal was to make a new game mode that doesn't take nearly as long to play a round as DotA does. And it's pretty much worked so far. It's a lot of fun.
One thing in particular I like is that a lot of under-used characters that are just underwhelming in regular DotA have made a comeback in the new mode, usually because their abilities are just better suited for the new map or something.
It's a lot of fun, and worth checking out. Especially since it's free. The only downside is that most of the community needs to be set on fire.
Edited, Sep 26th 2011 6:43pm by IDrownFish