The Pentacle
The Pentacle symbolizes the North, South, East, West and the Spirit. Second, it symbolizes the Earth, Water, Air, the Spirit and the divine. Third, it symbolizes the God and the Goddess. If encased within a circle, it means wholeness and unity of all quadrants and the elements and our ultimate love for Nature. Pagans and Wiccans do use the pentacle in their rituals.
Used as a talisman in the form of a pendant or engraved and used on an altar is to protect the person while banishing negative energies and auras.
The Pentagram
The Pentagram is actually a symbol of spiritual shielding and ethereal world intelligences. It is also a symbol of the mind dominating over the elements. The bad spirits and other negative elements are powerless when faced with this symbol and the good spirits are drawn to it.
The upside-down pentagram is not exclusively Satanic. In fact, there are some Wiccan covens that use the inverted pentagram as a way to denote ranking. It’s just the Satanists have adopted it.
History
It was the Sumerians who first utilized the pentacle symbol. For them it was a part of their language rather than a religious symbol.
Satanists do indeed use it as a symbol, typically with two points up and three down. They refer to this as the Sigil of Baphomet and can be used as a symbol for Leviathan.
It was also the official seal of Jerusalem for a while, showing its presence in Judaism as well in Christianity, it was used to represent the five senses. In medieval times, it was believed to represent the five wounds of Christ (two in the wrists, two in the ankles and one in the side). It's common use was actually to ward off demons and witches.
It has also represented the five fingers, the five joys Mary had of Jesus, as well as the five virtues of knighthood. However, it later became thought of as an evil symbol. Pagans and Wiccans use the pentacle in their rituals, as do Satanists, albeit in completely different manners. This led Christians to believe the pentagram to be an unholy symbol of evil, something that's carried forward to today.
Wicca and Witchcraft. The pentacle is a very important symbol in these religions. Many versions of the religion use it as their holy symbol, and wear it as Wiccan Jewelry, much like Christians have the Cross of Jesus and Jews have the Star of David. While not quite as universal as these other symbols witchcraft is one of its main associations.