In the Pokemon card game, there are two types of holo,or foil, cards. These differences are not hard to find, but may factor into how much the card is worth.
Holo, short for holographic, are cards with a shiny picture on them. In the Japanese version of the card game, certain rare cards only came in holo form. When the game came to America however, those rare cards came in both holo and non-holo variants; beginning with the second expansion, Jungle.
Reverse holo, or reverse foil, cards figure into the newer sets of Pokemon cards. In 2010, each booster pack of cards contains one reverse holo card, which can be almost any card from the set (excluding secret rares and Lv Xs). Reverse holo cards have a holographic or shiny background, instead of a foil picture.
As each reverse holo card is random, you can possibly receive two rares in each packs.
Other than the aesthetic difference, holographic cards are generally more valuable than non-holo cards. Among these, reverse holo cards are typically not too much pricier than their non-foil counterparts, because of the fact that one is in every pack. Regular holo cards are typically rarer to begin with, and thus, will probably sell for more.