Pentagrams and pentacles are very ancient symbols, used mainly in "the Isles" - Ireland, Britain, Wales, etc. Stars of Solomon (which look like the Jewish Star of David) and Egyptian Ankhs are also very popular symbols in our religion. However, pentagrams and pentacles are probably the most widely known and used.
A pentacle is the actual symbol, that of a five-pointed star with one point facing upwards. It is surrounded by a circle - the never-ending circle of life. A pentagram is the same symbol, but is usually a drawn image of the pentacle. However, many Wiccans use the two names interchangeably.
The five points of the star represent many things - it can be seen as a person, for example (the top point is the person's head, while the four other points symbolize the arms and legs), or as the five senses. The most widely used conception, however, is the elements - earth, air, fire, water - with the last point representing Spirit. It is a symbol of life and protection, and is worn as a sign of one's religion (just as a Christian would wear a cross). We also use the image on our altars (usually a disc made from materials of Mother Earth).
Unfortunately, Satanists took our symbol as their own and reversed it, with the point of the star facing downwards. This is one of many reasons people assume Wiccans worship the devil - they see the symbol, but do not understand the differences between our use and how Satanists have changed it. So, once and for all, if you see a symbol like this:
If you see a symbol with the star facing downwards, like this:
