In the Steam Marketplace, I've noticed some trading cards called "Foil Trading Cards". I can see from the price difference between an original and foil card that foil cards are much more valuable, but do foil cards actually do anything different than regular cards?
Steam Foil Trading cards can be crafted into foil badges, similar to how you can craft normal badges from normal trading cards. Other than that, it is useless, but everyone thinks they need one. They are essentially rarer trading cards. They are found the same way normal trading cards are.
From the Trading Cards FAQ: