Magic: The Gathering's Standard (formerly Type 2) format allows for the last two story-based blocks and the newest Core Set to be played. As of July 18, 2014, this includes Return to Ravnica, Gatecrash, Dragon's Maze, Magic 2015, Theros, Born of the Gods and Journey Into Nyx.
The Standard rotation is set to change in 2016. At that point, there will no longer be Core sets, and the blocks will only contain two sets. The first set of each block will cause the rotation. According to Wizards of the Coast, this will not substantially change how many unique cards are in the Standard card pool at any given time.