Commander is a rule set for Magic Online players, much like Two-Headed Giant or Emperor. It is based on the offline EDH rule set, which you can read more about here. Commander FFA is a variant of multiplayer free-for-all.
In Commander rules, each player has a starting life total of 40 rather than the usual 20. A player chooses a legendary creature, called a commander, and builds a deck including his or her commander. The commander is placed as the only card in the sideboard, and 99 other cards are put in the main deck. Only cards with a color identity that's a subset of the commander's color identity can be put into the deck. A card's color identity is the set of colors the card is and the colors of any mana symbols appearing on the card.
Before the game begins, each player's commander is put into the command zone. If a player’s commander would go to the graveyard or exile from anywhere, he or she may choose to return it into the command zone instead. A player may cast his or her commander from the command zone for its normal costs plus an additional for each previous time it has been cast using this rule. If a player would add mana to his or her mana pool that’s a color outside of color identity of that player's commander, that much colorless mana is added instead. In addition to the normal Magic loss conditions, if a player is dealt 21 or more points of combat damage from a single commander over the course of the game, that player loses the game. (The commander is tracked across zone-changes for the purpose of this rule.)
The following cards are legal for use in decks, but can't be used as commanders:
The current banned list for Commander is as follows: