Magic the Gathering is a trading/collecting card game that was created in 1993 and published by Wizards of the Coast. It is played between two or more players, each with a deck of cards he or she created. The rules are pretty simple to learn, but can get really confusing if the opponent has some older cards. It is currently played by about 6 million people. If you can't afford to buy cards (which can be really expensive, even from a specialized store), then there is a computer version available on Steam called Duel Of The Planeswalkers. If anyone has that, and wants to play a Player Match with me, do so within the next ten minutes, please.
EDIT: Personally I play a 3/4 Green 1/4 Red 60 card deck, but I'm thinking of making a Black deck, and pure Green mana ramp deck.