Players choose a legendary creature as the "Commander" for their deck.
A card's colour identity is its colour plus the colour of any mana symbols in the card's rules text. A card's colour identity is established before the game begins, and cannot be changed by game effects. Details
Cards in a deck may not have any colours in their color identity which are not shared with the commander of the deck. (The identity of each card in the deck must be a subset of the Commander's) Examples
A Commander deck must contain exactly 100 cards, including the Commander.
With the exception of basic lands, no two cards in the deck may have the same english name. Some cards (e.g. Relentless Rats) may have rules text that overrides this restriction.