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Standard is a rotating group of Hearthstone card sets (expansions or adventures). Most sets enter Standard when they're released and drop out 2 years later.
According to the offical blog post: "You’ll play Standard using a deck built solely from a pool of cards that were released in the current and previous calendar year, along with a core foundation of the Basic and Classic card sets (which will always be valid for Standard)"
The visual representation of the whole process: