The game is a 6v6 DotA style map that pits fleets of capital ships against each other as fighter ships push different lanes and can be killed for minerals, used to upgrade and outfit each player's ship.
If you're curious about this map, go ahead and watch this YouTube overview created by TotalBiscuit. If you want to play, simply find the game in the Custom Game list and hit join.
The game requires a lot of teamplay and features permanent death, which usually means that people will ragequit quite early. This is easily the game's biggest issue at the moment as people figure the game out, but development has been quite active and hopefully this issue will be addressed in the future.
Meanwhile, we're still waiting on the four custom maps from Blizzard, debuted at BlizzCon 2010, which included an internal version of DotA.