From Wikipedia: There are two main styles of a hamsa hand: the stylized hamsa hand with two symmetrical thumbs, and hamsa hands that are not symmetrical and shaped like actual hands. Either hamsa hand can be worn with the fingers pointing up or down.
The hamsa is popular as a charm most often worn as a necklace, but can be found as a decorative element in houses, on key chains, on other jewellery items. Many artists use the image of the hamsa hand in jewelry, paintings, sculptures, wall decorations, and amulets.
The renewed interest in Kabbalah and mystical Judaism is a factor in bringing the hamsa pendant back into vogue. In Jewish mysticism, fish are a symbol of good luck, so many hamsas are also decorated with fish images.