Commander (aka Elder Dragon Highlander or EDH) is a magic-the-gathering variant format which emphasises social interactions, interesting games, and creative deck building.
The primary characteristic is that decks are centered about a Legendary Creature as the Commander and the Commander's color attributes dictate what other cards may appear in the deck.
Decks must consist of 100 cards including the Commander, and duplicates are prohibited (with the exception of basic lands). It also introduces the "Command Zone", where you can choose to move your commander if it would otherwise go to exile or the graveyard. You can cast your Commander from the command zone, but the casting cost increases by 2 colorless mana each time. Commander can be played 1-on-1 but is usually played with more.