From Bulbapedia, the community-driven Pokémon encyclopedia.
A reprinted card is a card that has been printed in a previous TCG set and has been included in a newer set. Often times, basic Trainer cards are reprinted, particularly when a new block of sets is started. When a card is reprinted, older versions of a card can still be used in modified tournaments provided the text of the card has not changed. If the text of a card has changed only slightly, an official Card-Pokédex entry or a copy of the card with the most recent wording must be supplied by the player using the card.
Often, Ultra-Rare cards (LV. X, EX, etc.) are reprinted with different art for Collector Tins, becoming Black Star Promo cards as opposed to Ultra-Rare. Some examples are Charizard G LV.X and Darkrai-EX. Occasionally, the cards reprinted are not Ultra-Rare, such as Pikachu and Serperior.
Commonly reprinted cards