The Wiccan star, or Pentacle (or Pentagram when represented in a circle) is Normally a symbol of protection. Wiccans however are VERY eclectic people and each individual has thier own view on this. Traditionally the four points represent the four elemnts (Earth, Air, Fire and Water) and The point at the top represents our spirit, or any dieties being honored. A good article to read on thi: http://www.witchvox.com/va/dt_va.html?a=usma&c=basics&id=2875

It is a pentagram or a pentacle - depending upon whether or not it is encompassed with a circle - can mean a variety of things to Wiccans. The most common is the Elemental Pentagram. The Elemental Pentagram starts with Spirit (or Akasha), which is generally placed pointing upwards at the "top." The other points represent (counterclockwise around the points): Air, Earth, Fire, Water. Some Wiccan traditions use pentagrams of different orientation to denote the number of degrees an initiate has obtained. Other pentagram use includes drawing (generally in the air) an elemental invoking pentagram when the quarters/elements are called/invoked in a circle. The difference in these pentagrams is basically the point at which one begins drawing them. Corresponding banishing pentagrams are drawn in the air when the quarters are released. An alter paten is generally a circular plate with a pentagram on it. These are used on Wiccan alters to represent the element of Earth. Those are pretty much the basics of Wiccan pentagrams. Pentagrams have a long history of association with The Mysteries, religion, ceremonial magic, etc, because it is a key part of Sacred Geometry. It has even been used in Christianity for, among other things, representing the 5 wounds of Christ. http://en.wikipedia.org/wiki/Pentagram

I would like to say that the Wiccan pentagram is not to be confused with the Satanic Pentagram. The Satanic pentagram is upside-down, three points facing down to symbolize our rejection of the holy trinity. The Satanic Pentagram or Star of Baphomet as I think it can also called, often features the head of a goat inside the star itself. The goat is Baphomet. The Pentagram also often features a ring around it and the ring may sometimes "Leviathan", who is one of the rulers of hell. I know this answer has practicaly nothing to do with the question but I thought it would be nice to get it out there :)

The Pentagram/Pentacle is far more than just "the Wiccan star". The Pentagram is one of the oldest symbols known to man. One of the earliest known common uses of the Pentagram was around 3500 BCE in ancient Mesopotamia where it was symbolic of imperial power. Amongst the early Hebrews the Pentagram was ascribed to truth and symbolized the 5 books of the Pentateuch, to the Gnostics (and the Freemasons today) it was the ‘Blazing Star’, to the Druids it was a symbol of Godhead, in ancient Egypt is was a symbol of the ‘underground womb’, and to medieval Christians it was attributed to the ‘five wounds of Christ’. The Christian Roman Emperor Constantine I used the Pentagram, together with the chi-rho symbol, as his personal seal and amulet, and in medieval times it represented truth and protection against evil and demons. In the legend of Sir Gawain and the Green Knight, the pentagram was Sir Gawain's glyph, inscribed in gold on his shield, symbolizing the five knightly virtues. Traditionally, the Pentagram with one point facing upwards, symbolized summer, and with the two points facing upward, winter. It wasn’t until the latter half of the 20th century that Satanic groups emerged using the Pentagram with two points upward as a symbol, but unfortunately it is this vision that sticks in many people’s minds. During the long period of the Inquisition, the pentagram was seen to symbolize a Goat's Head. In the purge on witches, the horned god Pan became equated with the Devil (a Judeo-Christian concept) and the pentagram, for the first time in history became a symbol of 'evil' and was called the Witch's Foot. The Moroccan national flag consists of a green Pentagram on a red background! In this case the Pentagram is considered to be the "Seal of Soloman". These days the Pentacle is used by many Pagans and not just those following the Wiccan beliefs.

To put in the easiest form: The 5 POints represent five aspects: Earth, Air, Fire, Water, and Spirit. THe elements of the world.

Pictures of what other answerers have explained: Left to right: 1. Chi-Rho cross (as referenced by rev_marty) 2. Pentacle with elements (as referenced by AR) 3. wounds of christ/ petagram representing man (ARB: rev_marty) 4. mendes pentacle/inquistion pentacle(rev_marty)/star of Baphomet(peyton) 5. Winter pentacle/stanic pentacle (rev_marty)