From Bulbapedia, the community-driven Pokémon encyclopedia.
A Half Deck is a Pokémon Trading Card Game format that is becoming a popular 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. This format also take less time, accommodating shorter attention spans for younger players.
The 2013 Summer Battle Tournament held from August to September in Japan featured Half Decks as one of the options of play in their Urgent Missions tournament.
There have been numerous official Half Decks released in Japan, though a few appear globally as well.