What is Standard format?

Standard is a rotating group of Hearthstone card sets. 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: