by Ashiok Sun Sep 30, 2012 8:31 pm
Commander is a legendary creature.
All cards in the deck must adhere to its colours (For example, a card that requires Blue mana (or produces Blue mana) for any part of it cannot be used with a Commander that is Red, White and Green).
You can only run 1 of each card except for basic lands.
Lands must all adhere to the colours of the commander in the colours they produce. "Any colour" lands are fine, fetchlands of any kind are fine, since they don't produce the mana themselves. Lands that state they produce a mana of a colour that isn't in the commander's colour spectrum aren't allowed.
Colourless cards are always fine, unless they involve the use or production of a colour outside of the commander's colours.
Oh and there's a banlist. But the official site seems to be down atm. If it's up at any point, you can find all the rules here:
http://mtgcommander.net/rules.php