Its a very popular mod for Warcraft 3 (sort of like how CS was a mod for half-life). Basically there are like 100 different "Heroes" and you are placed on two opposing teams on separate ends of the map. Each team has 5 people on it, and each person chooses their hero at the start. There are then 3 lanes which have towers guarding them between the 2 bases. Every 30 seconds a wave of creeps will spawn and charge the enemy base, meeting the opposing wave somewhere in the middle. If you last hit a creep you get the gold/xp for killing it. Basically the object is to level up, get gold, and then work with your team to destroy the other teams base. If you die you have to wait like 1 minute before it lets you respawn.