Magic: the Gathering can be played in many different ways, called formats. Each format defines a number of players, what cards are legal in the format, various additions or deletions of established rules of the game, and several other aspects in order to provide an enriching. Several of these formats are sanctioned through Wizards of the Coast's DCI organization, however there are also many variant formats which have been designed by player groups across the world.
Magic Online has also brought forth new formats, many of which have their roots in offline Magic formats.
Constructed formats involve players bringing previously-constructed decks with them to play with. Players then utilize this deck for all games that are played in the format.
Multiplayer formats encompass all formats in which games may be played with 3 or more players. Formats that are considered multiplayer include formats where two teams of 2+ players play together on a team, or where there are more than two players playing against all other players, or a combination of the two.