The official Pokémon website has announced the Play! Pokémon 2015 Season TCG Format Rotation. The rotation rules go into effect on September 3, 2014 but the 2015 season started on July 14, 2014. Additional expansions become legal three weeks after they are released in the United States.

Standard Format 2015

The Standard format for the 2015 season will consist of the following expansions:
Black & White Boundaries Crossed, Black and White Plasma Storm, Black and White Plasma Freeze, Black and White Plasma Blast, Black and White Legendary Treasures, XY Kalos Starter Set, XY, XY Flashfire, XY Furious Fists, McDonald’s Collection, Black Star Promo Cards: BW55 and higher, XY01 and higher.

The New Expanded Format for 2015

Starting on September 3, the new Expanded format will be joining the familiar Standard format as a legal option for sanctioned Pokémon TCG events of all sizes. This new format will allow players to use all expansions from Black and White on, so players will have even more options when designing a winning deck! Players should double-check what format is being used before deciding which deck to bring to their local events.

The Expanded format for the 2015 season will consist of the following expansions:
Black and White, Black and White Emerging Powers, Black and White Noble Victories, Black and White Next Destinies, Black and White Dark Explorers, Black and White Dragon Vault, Black and White Dragons Exalted, Black and White Boundaries Crossed, Black and White Plasma Storm, Black and White Plasma Freeze, Black and White Plasma Blast, Black and White Legendary Treasures, XY Kalos Starter Set, XY, XY Flashfire, XY Furious Fists, McDonald’s Collection, Black Star Promo Cards: BW01 and higher, XY01 and higher.

Source: http://www.pokemon.com/us/pokemon-news/play-pokemon-2015-season-format-rotation/