Created and popularized by fans, the Commander variant is usually played in casual free-for-all multiplayer games, although two-player games are also popular. Each player's deck is headed by a legendary creature designated as that deck's commander and determines which other cards can be played in the deck (while except for basic lands, each card in the deck must have a different name).
Appropriately enough for a format named after the legendary creature that's leading your team, your commander works differently from other cards in the game. Before the game begins, each player sets his or her commander aside in a special zone, the command zone. You may play your commander from the command zone for its normal costs plus an additional 2 generic mana for each previous time it has been played this way. If your commander would go to a graveyard, hand, library, or exile from anywhere, you may instead put it in the command zone; this is a replacement effect. If a player is dealt 21 points of combat damage from a single commander over the course of the game, that player loses the game.
This larger-than-life format was invented up in the wilds of Alaska, and its originators used commanders only from the Legends set, especially including the Elder Dragons such as Chromium and Nicol Bolas. Over time, the format spread. It became popular among judges, who would play it following a day's officiating at the Pro Tour or at Grand Prix events. It soon reached the mainstream and became a fan favorite in casual playgroups everywhere, from the kitchen table to Magic Online to the Wizards of the Coast headquarters!
Commander is a multiplayer variant of Magic: The Gahthering. It was created as a format specifically designed to promote social interactions between players. If you wish to get more information on classic Commander, we recommend you to go to the Commander website: MTGCommander.net.
But some players did not have enough Commander players around to be able to play multiplayer Commander games, while others wished to concentrate on two players for many personal reasons. That is why Duel Commander (which means Commander games between two players) developed a vast community of players throughout the world. Duel Commander requires specific decks and doesn't use the same cards as those which are commonly played in multiplayer.