The best explanation I can find comes from Eileen Holland's The Wicca Handbook:
Modern Wicca began in England in 1939, when Gerald Gardner was initiated into a traditional British coven by Dorothy Clutterbuck (Old Dorothy). He later broke the coven's seal of secrecy and published books about the beliefs and practices of British Wiccans, because he feared the religion would die out. This began what continues to be a groundswell of people converting to Wicca.
Debate currently rages over whether Wicca is a new religion, or the oldest of all religions. Some say that Wicca has been practiced continuously in Europe at least since the Ice Age. They cite paleolithic carvings of female figures, such as the Venus of Willendorf, as evidence of Goddess worship having been the origin of all religions. No, say others, Wicca is a neo-pagan faith, a 20th century construct.
Wicca is actually both, I think, and see no point in debating the issue at all. Modern witches follow in the tradition of our earliest ancestors and are the shamans and healers of the 21st century. We are priests and priestesss of the Great Goddess; we practice the ancient art of sacred magic in the modern world. Certainly witchcraft has changed over the millennia, but we still have much in common with the neolithic practitioner crouched before a fire, crushing herbs for a healing brew. Methods and tools may be different, but the intent is the same: to help and to heal, to honor the Mother in all that we do.
Witchcraft has adapted when necessary – we are only just emerging from the siege mentality that the Burning Times imposed upon us. We are in the process of learning how to live openly as witches again. Witchcraft has also evolved – we no longer slay the sacred king each year to ensure the tribe survives and flourishes. There are no more burnt sacrifices in Wicca, no shedding of animal or human blood to make spells work.
Every Wiccan is a priest or priestess of the Goddess as well as a witch. We serve her in whatever ways we are able to serve, according to our talents, abilities, and personal circumstances. Each Wiccan determines his or her own code of personal conduct and behavior according to the Rede, so you will find Wiccans who are pacifists as well as Wiccans who are professional soldiers, some who are omnivorous and others who are vegans.
Wicca is an Earth religion – an accepting, open-minded faith that celebrates diversity and considers us all to be children of the same Mother. Gender, age, race, sexual orientation, physical status, family background, or ethnic heritage are not important in Wicca. We are male and female, old and young, gay and straight, healthy and disabled, and of all colors. There are no reliable statistics on this, but it seems to me that there are about the same number of male and female Wiccans.
We collect no dues, have no central organization, no governing body, no supreme leader, no great high priestess who speaks for the Goddess. Our temples are gardens and forests, libraries and beaches, mountains and bookstores. Wicca consists simply of its witches and their collective beliefs and practices. It is a voluntary association of individuals who share one faith, but practice it in myriad ways. No one is born Wiccan – not even our children, for we expect them to choose their own spiritual paths when they are old enough to make such choices. There is nothing like a dress code, but many witches wear a pentacle. We have no dietary restrictions, but many witches are vegetarians.
Wicca is an organic religion, one that is evolving and emerging as a worldwide faith. It is growing rapidly, although we neither seek converts or proselytize. This is not a faith that knocks on your door. It is one to which you make your own way. Wicca is a way of life, a belief system that reflects itself in the ways we interact with the world around us. Personal integrity and respect for Mother Nature are important parts of the Wiccan way.
The Wiccan faith has two pillars – The great Goddess and a poem called “The Wiccan Rede.” The first step in becoming a witch is to find your way to the Goddess. The second step is to establish an ethical system in which to use her gift of magic. I am often asked how someone can get involved in the occult without being seduced by its dark side. The answer is that you must have an ethical belief system, one with which you keep faith.