The pentacle (five pointed star, surrounded by a circle) symbolizes the figure of man with outstretched arms and legs. It stands for an integral personality; human/Microcosm; endless; power of perfection. In witchcraft, a reverted pentacle represents the devil's goat, and the witches foot. It's a sign of reversal of man's true nature.
(The figure stems from the alchemy and represents the unity of five elements: earth, fire, air, water and ether/quintessence; thus it also is a symbol for the super-natural.)
Pentacles are also often associated with creativity, material possessions, and wealth.
In most Tarot-decks, Pentacles was (or is) one of the four suits of the Minor Arcana. In the Toth and Rider/Waite decks, the name has been changed into 'Disks.'