The Pentagram (or Pentacle) depicts a five-pointed star circumscribed within a circle. Typically one of the points is in the upward position and each "arm" of the start represents an alchemist's element: earth, wind, fire, water. The fifth arm represents the "fifth element" ("quintessence"). This fifth element is composed of "ether" and represents the spirit. 
Many people (perhaps wrongly) believe the Pentagram is associated with Satanic belief or worship, but the Satanic Pentagram is an Inverted Pentagram. Rather than one point at the top of the star, the Inverted Pentagram depicts two points at the top of the star and one at the bottom. In this fashion it also represents a goat's head (and vice versa). In the Inverted Pentagram, the spirit is below the elements. This may represent the idea of idea of putting the material world and thus carnal desires before the spiritual health.
Medieval Christians believed it to symbolise the five wounds of Christ. The pentagram/pentacle was believed to protect against witches and demons.
The pentagram figured in a heavily symbolic Arthurian romance: it appears on the shield of Sir Gawain in the 14th century poem Sir Gawain and the Green Knight. As the poet explains, the five points of the star each have five meanings: they represent the five senses, the five fingers, the five wounds of Christ, the five joys that Mary had of Jesus (the Annunciation, the Nativity, the Resurrection, the Ascension, and the Assumption), and the five virtues of knighthood which Gawain hopes to embody: noble generosity, fellowship, purity, courtesy, and compassion.
Probably due to misinterpretation of symbols used by ceremonial magicians, it later became associated with Satanism and subsequently rejected by most of Christianity sometime in the twentieth century.