Just as the name implies, metal leaf is made of actual metal. It is possible to purchase real gold, silver, and copper leaf, as well as "composition leaf", which is made of cheaper metals that simulate the look of gold, silver, and copper. Imitation leaf also comes in variegated color patterns.
Most polymer clay hobbyists use the cheaper composition metals, but some people suggest that real gold and real silver have a better look. It's probably wise to practice with composition metal leaf before you decide to invest in pricier alternatives. Aluminum leaf, which mimics the look of silver leaf, has the added benefit that it should not tarnish the way that "the real thing" does.
Metal leaf is ultra thin and light-- so light that the slightest puff of air can blow it away. Leaf usually comes in a booklet with sheets of tissue to separate the delicate pieces of leaf. Metal leaf adheres easily to the slight stickiness of raw polymer clay.