Wicca is an earth and nature-honoring religion that celebrates the cycle of the year and the duality of divinity. Wiccans celebrate eight holidays; two solstices, two equinoxes and four other holidays roughly equally spaced throughout the year. These holidays are called sabbats.
Wicca, called 'witchcraft' by some, was popularized (or perhaps invented) in the middle part of the twentieth century by Gerald Gardner. He wrote a book called Witchcraft Today that was published in 1954. In the book, he claimed to have discovered a survival of ancient pagan religions still being practiced today. Most practicing pagans today discount his claim to a survival, and celebrate rather a revival (or at least a best-guess reconstruction) of a pre-Christian religion.
Wiccans believe that the deity is both male and female and as such, honor both gods and goddesses that represent some aspect of the deity. Some Wiccans choose not to personify a deity at all, and speak instead of the 'creative force of the universe' or some similar phrase. Some women choose to follow a version of Wicca that downplays or ignores the god-aspect altogether, as a reaction against what they perceive to be the 'masculinization' of more traditional religions.
Wiccans may practice with a group, called a coven, or alone, in which case they are called 'solitaries'. There is no central Wiccan authority or canon, with each group or individual deciding what their practice consists of. Note that many of these groups, may claim to be practicing the only 'true' Wicca. One of the best overviews of Wicca was written with solitaries in mind; Scott Cunningham's Wicca: A Guide for the Solitary Practitioner.
Wiccans, and most other neo-pagans, believe, in common with many of the Eastern religions, that there is a universal force that exists within everything. Wiccans believe that this force can be manipulated through an act of will — this is the basis for Wiccan magic (or magick, the preferred spelling meant to distinguish it from a stage magician's prestidigitation.)
Wicca's basis for ethical behavior is in the Wiccan Rede, or rule: "An (if) it harm none, do what you will." The 'harm none' qualifier puts Wicca in the same camp with Buddhism and other religions that believe in non-violence. The Three-fold Law further states that whatever you send out into the universe, be it good or bad, you can expect to return to you threefold.
Contrary to common belief, Wiccans do not worship or even believe in the devil. Wicca recognizes no personification of evil. There are certainly people who wear black and call themselves Satanists and wear pentacles (a Wiccan symbol) but they are generally not considered to be Wiccans.
It is true many young adults and teens are initially drawn to Wicca as an act of rebelling against the religious faith of their parents. And Wicca is no more immune than any other faith to abuse by powerful personalities in need of followers to control.
The number of people who self-identify as Wiccan or other neo-pagan religion continues to increase — there are now Wiccan chaplains tending to the pagan population in prison and serving in the military.