From Bulbapedia, the community-driven Pokémon encyclopedia.
The half deck is a trend among Pokémon TCG players, particularly in Japan. In half deck play, both players use a 30-card deck instead of a traditional 60-card one and lay out three Prize cards instead of six. The format caught on quickly, in part because there is very limited space in most Japanese TCG Leagues and also because the games take less time and will be able to hold a person's interest longer. Although no major tournaments, either in the United States or overseas, are currently played in half deck format, there have been numerous official half decks released in Japan.