From Bulbapedia, the community-driven Pokémon encyclopedia.
Holofoil cards (also known as Holographic Foil, Foil, Holo, Reverse-Holo, Reverse foil or Holographic) are a special type of card from the Pokémon Trading Card Game. Most are Pokémon cards. They are identical to their normal counterpart aside from a holofoil effect in the picture (Holo) or the card background (Reverse-Holo). A small amount of Rare () cards within each expansion are holofoil. They may be pulled from Booster packs as the rare card, but is not guaranteed. They are rarer compared to non-holofoil rare cards.
In most of the English versions of Japanese sets, cards that were originally only holofoil are given non-holofoil counterparts. The e-cards are an exception to this, as even the Japanese versions of the sets had holofoil and non-holofoil versions of cards. Only the non-holofoil versions had dot codes that could be read with the e-Reader. This was apparently because the holofoil caused complications with the dot codes. When holofoil cards have non-holofoil counterparts, the non-holofoil cards are the more common of the two, even though both are marked with the same rarity. As such, they are numbered differently within sets and generally considered different cards in terms of collectibility. In play, however, the cards operate the same, and thus are viewed mostly as the same.
Both crystal and pearl figures from the Pokémon Trading Figure Game appear to be based upon this concept. However, unlike the cards, the figures are not numbered differently.
For a full list of cards with Holofoil prints, see here.