Just as the Horned God is considered an amalgamation of so many male horned deities from many different pagan religions, so is the Triple Goddess so many female pagan deities rolled encompassed in one being. The Goddess is considered triple because she is thought to represent three stages of womanhood.
I read on one page an author describe the phenomenon as something like the Christian Father, Son, and Holy Spirit all being distinct, but all being One. Of course, Christianity and Wicca are not the same religion, perhaps that analogy can provide some sort of starting point from which those of us unfamiliar with Wicca may begin to understand just like those of us unfamiliar with piping wouldn’t jump right into a job at Hamilton plumbing.
The three stages of womanhood represented by the goddess are the Maiden, the Mother, and the Crone. Naturally, each of these entities represents the various virtues and characteristics of different stages of a woman’s life. First, the Maiden, who is often depicted as being a virgin, is young and innocent. She is associated with newness, possibility, opportunity, fertility, the moon, and, of course, Spring. In the Triple Goddess symbol, shown above, she is represented on the left, by the waxing crescent moon. Furthermore, each of the stages of the goddess is associated with a Greek moon goddess. The Maiden is, naturally, associated with Artemis, the virgin goddess of the hunt.
The Mother is depicted as a mature, loving being, sexually experienced and nurturing. She is associated with stability, power, family life, domesticity, and with the summer when she is pregnant, and with the winter when she gives birth. Hers is the full moon, and she is thus associated with the loving mother goddess, Selene. Third is the Crone. The Crone is wise, strong, and respected, and is there to guide through some of the darker elements of life. She helps people deal with fear and destruction, with decay and change, and with death and rebirth. She is associated, of course, with the waning moon, and is paired with the goddess Hecate, “Queen of the “Witches,” associated with magic and the Underworld.
All that said, one must be careful to not always attempt to clump all goddesses into these three categories; some may fit into more than one, and some may not properly fit into any, and that’s okay. Along with many other neopagan religions, Wiccans believe that women may embody the goddess by engaging in exclusively feminine activities, such as breastfeeding, pregnancy, and other sexual reproductive acts. These things make the human body sacred. The Triple Goddess, often referred to simply as the Goddess, or sometimes, as I understand, the Lady, is Mother Earth, and mother to us all. In the Wiccan faith, even the Goddess’ male counterpart, the Horned God, as he dies every year in the autumn, is once again reborn at Yule not according to his own volition, but from the Goddess, for everything springs from her.