@arl: Well, to be technical, DOTA doesn't stand for anything. It was originally DotA on Warcraft 3 which was short for "Defense of the Ancients" referring to the fact that you - as a team - defend your "ancient" which was either a world tree or the frozen throne. Due to copyright or whatever, Valve couldn't actually use the name "DotA" so they went with "DOTA" which was obviously related, but it doesn't actually stand for "Defense of the Ancients."
It's an AoS genre which basically means there's NPC monsters spawn from both teams into three different lanes. You get EXP by being within a certain radius of a dying enemy (whether player-controlled or NPC), and you get gold for killing enemy players, helping kill enemy players, and getting the last hit on a NPC monster. You level up your skills, you buy items, you kill the enemy team, and you ultimately try to destroy their base. That's basically it.