The symbol can actually trace its origins back to the great religion of mathematics. Historians believe that in ancient Sumer, the points represented the four cardinal directions (or at the very least forwards, backwards, left, and right) and the fifth represented elevation. Pythagoras himself saw the symbol as "mathematical perfection."
Probably the most popular use of the symbol is in Paganism, where it has long represented the four elements and the spirit. While most Christians would equate this with Satanism, the truth of the matter is that Pagans do not believe in hell, and therefore do not believe in Satan. While Wicca might reference a Horned God, he is about as satanic as Pan was.
Even more interesting is the fact that Christianity itself has used the symbol in its history. I remember having seen five pointed stars adorning the Lutheran Churches I attended as a child. In an ironic twist, many believed the Pentagram would protect them against witches and demons, the very things they think it symbolizes today. Others used the five points to represent the five wounds of Christ.
In fact, in Satanism, their symbology is very specific. The Satanic pentagram features the star pointing downwards, transcribed in two concentric circles, with the head of the goat filling out the star. (They were not going for subtlety here).
The list of religions and cultures that have used the pentagram in some aspect of their mythology goes on. I find it interesting the the same symbol can have so many different meanings being beloved in some instances and reviled in others.
(All info from wikipedia).