From Bulbapedia, the community-driven Pokémon encyclopedia.
A Supporter card in the Pokémon Trading Card Game is a type of Trainer card that is usually especially powerful. As a result of this, a player may only play one Supporter card each turn.
Supporters first appeared in the Expedition Base Set, and instantly were included in most decks. A Supporter card, when played, will remain on the playing field until the end of the player's turn, rather than going directly to the discard pile after use.
Since Diamond & Pearl, Supporters became their own class of card, disallowing cards that do not explicitly tell the player to search for a Supporter card from seeking these cards from their deck or discard pile. In Unlimited format games (as all older Trainer and Supporter cards have since been rotated out of Modified format), older cards that refer only to "Trainer cards" in their text may be used to search for Supporter cards as well as Stadium cards.
Many veteran players who had been playing the game since its early years were critical of Supporter cards when first released, the main complaint being that most, if not all, cards that allowed players to draw cards were Supporter cards that could only be played once per turn, which would slow down decks and increase the amount of luck needed to successfully run a deck.
In other languages
Brazilian Portuguese: Apoiador (Suporte is also seen)
German: Unterstützer (Supporter is also seen)