What is the Noble format?
At its core, it is a hybrid of the Peasant and the EDH/Commander formats. Contrary to its name, Noble is designed for less-than-affluent players who have great ideas but don't have the money to buy a deck loaded with top-tier cards. Rather, it is designed for people who have a few good 1-ofs, but don't have enough money to fund competetive decks in other formats. The general idea is to build around a favorite card or just a card that you have a great idea for.
Noble decks may include:
Regarding rarity, each card's effective rarity is based on the lowest-rarity printing of that card. For example, Oblivion Ring is considered common even though it was printed as uncommon in the Magic 2012 Core Set. Furthermore, a card like Serra Angel that was printed as a rare at one time cannot be used as your Noble Card because its lowest rarity printing was uncommon.
Beyond these rules, deck construction is the same as traditional deck construction. Specifically, there is a 60-card minimum, and this includes your 1 rare/mythic, up to 4 uncommons, and 55+ commons.
Noble is an eternal format, but it can be restricted to exist in any of the non-eternal formats like Standard or Modern.
SideboardingSideboarding is optional. You may choose to build a sideboard of up to 15 cards. Any cards contained in your sideboard count towards your overall rarity quotas. For example, if you choose to run 1x Path to Exile in your sideboard, you may only have up to 3 other uncommons anywhere in your deck. No rares or mythic rares are allowed in the sideboard, since only your Noble card is allowed to be rare or mythic rare.
Okay, now how do I play?Separate your Noble Card from the rest of your cards and set it aside. Much like the EDH format, your Noble Card begins face down in the Command Zone, where it cannot be interacted with. You may look at your Noble Card at any time, and you may cast your Noble Card at any time allowed by the standard game rules.
Your Noble Card may be cast from the Command Zone as though it were in your hand, and any abilities of your Noble card may be activated from the Command Zone as if the card were in your hand (for example: Suspend, Forecast, Ninjutsu).
Once your Noble Card has been cast, it is treated as a normal card, and it cannot be returned to the Command Zone.
Beyond these rules, the game proceeds as any normal game of Magic would.