In many modern Pagan traditions, the triple goddess in the form of Maiden/Mother/Crone is honored. She is seen as the feminine counterpart to the Horned God, the female who provides polarity to the male essence. In some traditions, such as many Dianic Wiccan groups, the triple goddess is the only deity worshipped.
It's important to remember that the concept of a single goddess representing the Maiden/Mother/Crone is primarily a Neopagan and Wiccan one - most ancient cultures did not have a Maiden/Mother/Crone figure, although they did include other triune or triple goddesses. The contemporary notion of the Maiden/Mother/Crone was popularized by folklorist Robert Graves, in his work The White Goddess. Graves theorized that there was an archetypical triad of goddesses found in the mythology of various European cultures. However, much of Graves' scholarship has been discredited due to lack of primary sources and poor research.
In modern Wicca, however, and many Pagan religions, the Maiden is seen as the virginal young woman, or girl, who has not yet awakened. She is all about enchantment and new beginnings, youthful ideas and enthusiasm. She is associated with the waxing phase of the lunar cycle, as the moon grows from dark to full.
The Mother is the next phase in a woman's life. She is fertility and fecundity, abundance and growth, the gaining of knowledge. She is fulfillment -- sexual, social, and emotional -- and she is represented by the full moon. Springtime and early summer are her domain; as the earth becomes green and fertile, so does the Mother.
In some forms of feminist spirituality, the Maiden/Mother/Crone is used as an example of society's treatment of women. While the Maiden is revered and the Mother is honored, the Crone is pushed aside and reviled. Many women are trying to turn that around and reclaim the title of Crone, much like the gay community has reclaimed "queer." Instead of allowing themselves to be "old ladies" at Cronehood, these women are taking back the notion that with age comes wisdom. They are vibrant, sexual, life-embracing women who are proud to be labeled as Crone. Instead of hiding in the shadows, they celebrate the later years of life.