Each player is a magician/sorcerer/whatever with 20 life points, and the object is to take those points away from your opponent.  You can do this directly with magic attacks, or indirectly by attacking with creatures you control.  You can summon creatures or cast spells by playing cards, and there are like 5,000 different cards now.  Each card changes the rules of the game, and the combinations of cards and sheer variability of the rules makes the game insanely complex.