Duel Commander is a variant of Commander format meant for two-player games.
Decks consist of 100 cards and no sideboard, one of which is a legendary creature chosen as a commander.
Except for basic lands, all cards are allowed in only 1 copy. No card is allowed if its color identity contains colors outside color identity of the chosen commander.
For more information and detailed deck construction rules, visit the official Duel Commander website.
Duel Commander has "restricted" cards, which are allowed in the deck, but not as a commander.