Winner cards are reprints of select cards from various Pokémon Trading Card Game Expansions with a foil "WINNER" stamp in the bottom right corner of the card art. These cards were released through Pokémon League or related tournaments.
The Wizards-era "Winner" stampThe Nintendo-era "Winner" stamp The Winner stamp was first portrayed on cards from the Best of Game collection and were given to Pokémon Professors and Tournament Organizers of the BattleZone tournaments. The stamp itself is a gold foil "winner" in lowercase with a star for the point in "i".
They later appeared again as prizes within the Pokémon League after Nintendo acquired the TCG licence, this time from a variety of sets. The stamp was updated to match the stamp featured on Prerelease cards, with a Poké Ball symbol to the left of "WINNER".
The Winner cards were eventually phased out when stamped cards for Premier Events were produced under the new system introduced by Pokémon Organized Play.
Trivia
It is possible that the idea of the Winner stamp was conceived from the "W" Promotional cards, which Wizards had produced early on in the TCG.