From Bulbapedia, the community-driven Pokémon encyclopedia.
Salamence Booster pack from the Arceus
Booster packs are packs of 9 to 11 cards in the Pokémon Trading Card Game. Boosters offer the chance to acquire stronger cards to strengthen a Theme Deck, create new decks of cards or customize old ones. Packs contain common (), uncommon () and rare () cards, and occasionally "super rare" cards such as Pokémon-ex, Pokémon Star, Shining Pokémon, Pokémon LV.X or Pokémon LEGEND.
Booster packs have contained varying amounts of cards over the years. From the first Base Set expansion through to Neo Destiny, packs contained 11 cards. Beginning with the e-Card series and continuing through the entirety of the EX series, packs had only 9 cards. On the release of the first Diamond & Pearl era expansion, the pack size was increased to 10.
Every booster pack is guaranteed to contain at least one rare card, with generally three uncommon cards and the remainder being common. However, the rare card can have a rarity of rare (★), holographic rare, or be a "super rare" card mentioned above, such as a Pokémon-ex. Since the release of the Legendary Collection set, one reverse foil print of a common, uncommon or rare card has also been included in every pack. A reverse foil card is identical to its normal counterpart aside from a holographic foil effect in the picture or card background. Reverse foil cards in the EX series also contain a stamp of the set logo in the bottom right corner of the picture. The inclusion of reverse foil rare and holographic rare cards created the potential that one pack may contain two rare cards.