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 era of the TCG begins. 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 at all, 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
Throughout the course of the TCG, some trainer cards have received functional errata due to being reprinted. This happens when a card is reprinted with the same name, but a different effect. Unfortunately, due to the Unlimited format not being supported by Pokémon Organized Play, and the lack of information as to how many of these cards should be treated or used in the Unlimited format, it is unclear if they are true reprints, or different cards entirely. Notable cards that have received functional errata through reprints include:
Great Ball is especially notable, as it has received 3 different printings, each with a different effect. Although PlusPower has had 4 different wordings throughout the history of the TCG, only one of them actually changed the functionality of the card, changing (after applying Weakness and Resistance) to (before applying Weakness and Resistance). Although Bill did not receive a change of its text, it was changed to a Supporter card, thus altering its in-game effect.