The pentagram or rather five point star, is the symbol of venus (the planet itself). But venus has the symbolism of the Queen of the Heavens and therefore can be used to symbolize the Goddess, or a specific goddess that is associated with the planet, such as Inanna, Astarte, Aphrodite, Artemis, Isis, etc.
A pentagram is a star alone.
But more popular is the pentacle--the star within a circle. People have placed many various meanings to it, from a man outstretched, to the four elements + spirit/akasha, to merely that of protection. But primarily, it has become a symbol to identify pagans that practice magick. You will generally find at least one on a magick practioner that honors nature.