From Bulbapedia, the community-driven Pokémon encyclopedia.
Energy cards (Japanese: エネルギー Energy) are the cards that power Pokémon attacks in the Pokémon Trading Card Game. Without the required Energy cards attached, a Pokémon cannot use these attacks. Energy cards come in Basic and Special forms.
Basic Energy cards
Basic Energy cards (Japanese: 基本エネルギー Basic Energy) are defined as eight Energy cards that share their printed type with a Pokémon. They are the only cards that players are allowed to have more than four of in their decks. Basic Energy cards always provide their printed type of Energy, as well as Colorless. However, there is no basic Energy card yet that provides only a single Colorless Energy.
Special Energy cards
Special Energy cards (Japanese: 特殊エネルギー Special Energy) are cards that provide more than one Energy of a specific type and/or have an additional effect besides providing Energy. Some may heal the Pokémon they're attached to, add damage to their attacks, or may even provide several different types of Energy at once. A majority of them provide only Colorless Energy; however, there are several which provide one or more of any type of Basic Energy.
For a full list of these, please see List of Special Energy cards.