Despite being part of legal sets, the following cards are explicitly not allowed in decks for this format.
Standard is a tournament format containing several recent Magic: The Gathering sets. Most sets enter the format when they're released and drop out about twenty-one months later.
Generally the group contains 5–8 sets; when a ninth would be released, the eldest four are dropped. This is a rule of thumb and exceptions are frequently made. This site will always have current information.
A Standard deck contains 60+ Standard cards and can optionally have a sideboard of up to 15 additional such cards. Apart from basic lands, the combined main deck and sideboard cannot have more than four copies of any card.
Brawl is a format based on Standard—all rotations listed here apply to Brawl as well—but Brawl has its own banlist.
Not all sets enter Standard upon release. For example, Masters sets and Commander sets never enter the format.