Standard is a rotating group of Magic: The Gathering sets. Most sets enter Standard when they're released and drop out about eighteen months later.
A deck which contains only Standard cards is called a Standard deck.
The set rotation pattern for Standard is currently in a two-year transition between the old set release pattern and the new one.
The Theros block and 2015 Core Set were the last group to rotate out in the old pattern.
The Khans of Tarkir block and Magic Origins, although released in the old pattern, are dropping in the new: Khans of Tarkir and Fate Reforged dropped together in April as if they were a two-set block; Dragons of Tarkir and Magic Origins will drop in the same fashion Q4 2016.
After that, all sets will be both released and dropped in the new style.