Unlike leaf, which is real metal, transfer foils-- even those that appear metallic-- contain no metal. Instead, they are sheets of colored plastic with a clear layer of plastic (referred to as "backing") on top. Foil comes in a variety of colors, many of them metallic. Some are holographic or patterned, either in geometric shapes of in abstract swirls of iridescent color.
While leaf sticks to uncured clay on contact, foils take more effort to apply. Some are easier than others to transfer. The ones that seem to work best are heat-sensitive, rub-on foils that can be burnished onto the clay. Those that require a special adhesive are not as likely to work well with polymer clay.
To apply foil, place it on the clay with the colorful, shiny side facing up. Then, depending on the type of foil, you should thoroughly burnish (rub) it and/or heat it with an embossing gun, a hair dryer, or an iron. Once you think the foil has transferred, rip the plastic backing away from the piece. It may take some practice (and persistence) to achieve the desired result, and there are different techniques, so check around and experiment until you find one that works for your particular situation. (There is a great deal of information on this subject at Glass Attic: Leaf and Foils -- http://www.glassattic.com/polymer/MainPages/leaf.htm.)
While transfer foils are more difficult to apply than metal leaf, they do have their benefits. Because they are not made of metal, there is no risk that they will tarnish. They don't even have to be sealed, though it's probably a good idea to do so if the object will get heavy wear. Foil also comes in a wide variety of colors and effects not available in metal leaf.
Foil can be used with polymer clay in many of the same ways that metal leaf can. For instance, it too can be crackled or used in mokume gane. Because of its unique colors and patterns, foil is also a popular choice for people making faux dichroic glass.