DOTA or Defense of ancient is a freemap or an edited map of warcraft 3:Frozen throne where you defend your ancient(World tree for the sentinels and Frozen throne for the scourged). You pick a hero where you fight creeps of the enemy and face your enemy's heroes also played by other players or ai. you have to make strategies on killing and making your items.
Heh, you guys must not like WC3 then. DotA is one of the biggest custom games to be played on Battle.net. It gets hectic with 8 people to play against =)