DotA stands for Defense of the Ancients, it's a Multiplayer online battle arena where you and 9 other people select a hero and have to level up and buy items to destroy each other's main tower.
There are 3 types of heroes but they all fill different roles, Strength, Agility and Intelligence.
Strength heroes have high health and medium damage, they're basically tanks.
Agility heroes have high armor and high damage, usually with some lifesteal, they're the physical damage dealers.
Intelligence heroes have high mana pools and damaging or "buffing" spells, they're usually supports or carries.
http://www.youtube.com/watch?v=54g5ej4NAek
http://www.youtube.com/watch?v=1GHYU7zv2Us
http://www.youtube.com/watch?v=R9-qHJGIUvU
Those 3 videos are for handicaps people that don't know how to play.
It's like DotA, but with a 2 at the end.
Discuss the game here.
Pretty sure there is no NDA anymore, so.
I play Pugna alot, I get a Dagon, Phase Boots, Argagagagahslayu's Scepter and then max Dagon and get strength items for HP.
Then all you have to do to kill someone is Dagon's Active -> W -> Q -> R until they die.