Our main symbol
Many religions, even those involving a variety of symbols, hold one above all others to represent the religion. For Jews it is the star of David. For Christians it is the cross, although they employ dozens of variations of the basic design. For Wiccans, it is the pentagram.
The pentagram has existed for thousands of years through a variety of cultures. It has been ascribed numerous meanings, but mostly it has been a symbol of protection and invocation.
As a symbol of our religion, the pentagram is always depicted point-up, and many Wiccans will rabidly insist it is only used in this orientation, thus further distancing ourselves from Satanists, who identify themselves was a point-down pentagram. However, there are in fact some Wiccans (and witches) who use pentagrams oriented in both directions, representing a variety of different meanings.
The pentagram symbolizes the union of the five basic elements. Spirit is ascribed to the uppermost point, with the implication that Spirit surmounts the physical elements. When the pentagram is inverted, the physical world supplants the Spirit as supreme, hence its association with Satanism. Personally, it seems to me a certain point is lost when you focus on the position of any one element among a unity.