Standard is a rotating group of Magic: The Gathering sets. Most sets enter Standard when they're released and drop out about twenty-one months later.
Generally Standard will grow until it would receive its ninth set, then instead it drops its oldest four sets to receive it as a fifth set. There are exceptions to this. It's confusing.
A Standard deck contains only Standard cards (60+ of them), can optionally have a sideboard of up to 15 more Standard cards, and apart from basic lands does not have more than four copies of any one card .
Brawl is a format based on Standard—all rotations listed here apply to Brawl—but Brawl has a separate banlist.
Not all sets enter Standard upon release. For example, Masters sets and Commander sets do not enter Standard.
Despite being in Standard sets, these cards are explicitly not allowed in Standard decks.