At the most fundamental level, a Goddess is a God who is female. Many cultures have worshipped goddesses, sometimes alone and more often as part of a larger pantheon which includes both gods and goddesses.
Wiccan and Neopagan worship includes worship of The Great Goddess along with the Horned God. There has been some association of the Wiccan Goddess with the Greco-Roman goddess Aradia, also with Mother Earth (or Mother Nature), and with the moon. She may also be refered to as 'Queen of Heaven' . The Great Goddess is believed by many Wiccans to have been worshiped in a universal pre-historical matriarchial religion. The Goddess is worshipped sometimes by worshipping her different aspects, represented by individual goddesses like Isis, Kwan Yin, Kali, Pele or Athena. The yoni or vulva may also be revered as an aspect or symbol of the Goddess.
The Goddess is worshipped in three forms; as Maid (or Virgin), Mother, and Crone. The three forms represent the stages of life. The moon and the circle are considered her symbols. Often the three stages of the moon (waxing, full, waning) are considered equivalent to the three forms of the Goddess, and put together in a single symbol. The fourth stage of the moon, the dark moon, is sometimes considered a symbol of a fourth stage of the Goddess, the Dark Goddess.