Commander also known as EDH is a casual eternal Constructed multiplayer format.
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 Commander website.
Commander formerly had cards "banned as commander", which were allowed in the deck, but not as a commander. Currently there are none, but there is a card "banned as companion", which is allowed in the deck and as a commander, but not as a companion.