pente is Greek for five, and gram is Greek for "to write". So, the word pentagram actually means any figure of five lines

An upright pentagram is a 5-pointed star with one point up.

An inverted pentagram is a 5-pointed star with two points up. Usually associated with Satanism.

An upright pentacle is a 5-pointed star with a circle around it. Usually associated with Wicca

Upright pentacles and pentagrams are among the most widely used religious symbols. They have been used in many eras and by many cultures and religions of the world: by ancient Pagans, ancient Israelites, Christians, magicians, Wiccans and others

Apparently (or from what sources say) it's use goes ways back to as the symbol of the Goddess Kore (Carmenta,Ceres...depending on which culture you study). Her sacred fruit was an apple, and if you cut one sideways, you will see their is a star in the middle. They gave this Goddess many props, including creating the Roman alphabet. They had festivals in her honor and gave her holidays (which was not uncommon for our Pagan ancestors to do).

The Celts believed that it was the sign of Morrigan..."Ireland had five great roads, five provinces and five paths of the law. The fairy folk counted by fives, and the mythological figures wore five fold cloaks."

During the times of the old testament (Hebrew times), the pentacle was the first and most important of the seven seals - an amulet whose seals represented the seven secret names of God.

In Christian times:

The five points represented the five wounds of Christ.

It was used to symbolize the star the led the priests to the baby Jesus.

It has been referred to as the star of Bethlehem

It has been widely used by Christians as a protective amulet

Roman emperor Constantine used the symbol in his seal and amulet

King Arthur's nephew took it as a personal symbol and had it place on his shield. The five points symbolized "the five knightly virtues - generosity, courtesy, chastity, chivalry and piety

Many religious and spiritual groups use the pentacle or pentagram as their symbol. But most widely known is Wicca, in which this is their main symbol. Wicca came into play just in recent years (the 50's, which doesn't seem recent to you or I, but actually is given the age of religion itself) and is a reconstruction of Witchcraft... from the imagination of (mainly) two men and the bits and pieces we have been able to gather about our Pagan ancestors and their worship. We (wiccans) usually wear this symbol to show our religion, much the same as others wear a cross. We also sign the pentagram in the air for blessing ourselves, others or objects... much the same as a catholic signs a cross on their chest. We also use it for invoking and banishing elements and such. And this is pretty much our meaning of it....the elements that is.... The five elements that sustain life... Earth...Air...Fire...Water...Spirit...

Was that a little more information than you were looking for? I guess I could have just summed it up with that last line, hah?