WotC has taken Elder Dragon Highlander, or EDH, a popular grass-roots multi-player Magic format, and turned it into an officially sanctioned format. They have renamed it MTG Commander.

In this format you have big-ass legend as a commander. You can only play with cards in your deck of the same color as the commander, the commander can be cast as if it was in your hand, and it goes back to your reserves if killed. The minimum deck size is 100 cards, you can’t have more than one copy of a card, and everyone starts with 40 life.

Along with this format, WotC will be designing more cards specifically for multi-player Magic.

(source)