From the official Pokemon website: 

In the Pokémon Trading Card Game, players build decks around their favorite Pokémon and then play against each other, sending their Pokémon into battle with the goal of winning Prize cards by defeating their opponent's Pokémon. Players can build their decks from scratch or begin with theme decks—preconstructed decks designed to cover the basics of the game. Then, they can augment their card collections with booster packs that provide more cards, letting players develop more diverse decks. With thousands of cards to choose from, the game is never the same twice. New sets of cards, called “expansions,” are released throughout each year, so the game continues to evolve and expand for both players and collectors.