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 recently dropped out of Standard together, following the old rotation pattern.
The Khans of Tarkir block and Magic Origins, although released in the old style, will drop out in the new; Khans of Tarkir and Fate Reforged will drop out together as if they were a two-set block, followed by Dragons of Tarkir and Magic Origins in the same fashion six months later.
After that, all sets will be both released and dropped in the new style.