Edit:
Warcraft III and World of Warcraft aka WOW are two different games. If TF2 and CS:S are like potato, potato [poteito, potaato], then Warcraft III and WOW are like tomato, cucumber [tomeito, qcumber].

Defense of the Ancients (DotA) is a custom made map to Warcraft III, which can be downloaded here:
http://www.getdota.com/It is somewhat balanced and the choice between 95 different heroes, (who are actually much more unique than one would expect,) makes for lots of variation from game to game.
It is an up to 5v5 teambased competitive game, where one team play as The Sentinel, and the other as The Scourge.
The goal of each game is to wreck the other teams Tree of Eternity/Frozen Throne.
To get from one base to the other there are three main lanes, each guarded by three defensive towers, that must be destroyed in order before reaching the base.
Every 30 seconds spawns both teams a small creep wave in all three lanes, that the players should kill/protect in order to take down enemy towers.
When heroes kill* creeps or enemy heroes they gain experience and may advance up to level 25.
Heroes are also get gold from killing** creeps, heroes and building, which are used for purchasing different items.
*Only need to be in proximate range to actually get xp.
**Last-hitting
DotA is a game where more knowledge about hero-skills and items tend to be as important as how you actually use your hero ingame, as long as you can utilize those long hours of reading about heroes and items.
Also it is not necessarily the best ganker* hero's team that wins the battle, because at a certain point his hero will stop leveling, with the remaining heroes catching up to him.
And of course it is a teamgame, somewhat like TF2 without the possibility of changing class.
*Killer, dominating, God-like, carry
As online play tend to be somewhat laggy, using a program like
www.hamachi.com can set up Local play over the internet.
Anyone plays it?