From Bulbapedia, the community-driven Pokémon encyclopedia.
A rotation in the Pokémon Trading Card Game is when several sets are removed from Play! Pokémon sanctioned events, generally because they are too old and would not be compatible with newer sets or rules. Rotations almost always occur directly following the World Championships and remove four or more sets. If a set is rotated, that means a player may no longer use any card from that set in their deck (with exceptions; see below). This rule only applies to Standard format (formally referred to as Modified format) and Expanded format. In Unlimited format, players may use any card from any set in their decks.
Legal cards from rotated sets
Some cards in sets that are part of the current rotation are reprints of cards that appeared in older rotations. If a card is included as part of any set in the current rotation, any version of that card is legal for tournament play. For example, at least one version of Rare Candy has been legal in every modified format since it was printed. Thus, older versions of Rare Candy, even those from rotated sets like EX Sandstorm and Great Encounters, are still legal cards. Similarly, basic Energy cards from all sets are still allowed in tournament play.
In cases where a card is reprinted with significant text changes, the player playing the card must have a new copy of the card on hand, separate from their deck to serve as reference. For example, the effect of Energy Retrieval printed in Black & White is different from the original print in Base Set. A player would be permitted to include the Base Set print of Energy Retrieval in their deck for tournament play, but would be required to provide a copy of the Black & White print as reference.
Expansions that are part of the current rotation
Expansions that have been rotated out or were never legal