What is Wicca?
Wicca, which may alternatively be referred to as Witchcraft (which is actually an older form of magic and not a belief) the Old Ways (an inaccurate assumption of the faith's age) or sometimes as simple the Craft, is one of the most popularly practiced neo-pagan faiths on the market. As such when you're out shopping for a religion (whether you're a malcontented teenager or an adult going through a mid-life crisis) you're quite likely to come across Wicca as one of the options you have. However Wicca can sometimes be difficult to get a handle on for the very reason it's so popular; customization and decentralization.
Let's begin at the beginning. Wicca was created sometime between the late 1930's and the 1950's (see more on that here for additional details because it's a fairly long story), and it was popularized by a man named Gerald Gardner. Gardner wrote a fictional account of pagan faiths that had survived into the modern day entitled "High Magic's Aid." However, most people agree that because so many pagan faiths were stamped out with no written records of myths, doctrine and beliefs that the idea of entire faiths surviving is fairly ludicrous. Rather, Wicca is seen as a re-invention of the spirit of pagan, European faith with some best-guess scenarios thrown in for the ceremony and rites involved.
Wicca is a decentralized faith, meaning that unlike Christian religions there is no Wiccan church, or group that oversees Wiccans to make sure they're abiding by the rules. In fact one of the only rules that's embraced unilaterally by Wiccans (most of the time anyway) is the Wiccan Rede; a simple phrase that goes something like, "An it shall harm none, do as you will." This throws in a touch of peaceful, Eastern philosophy into a churning pot that has very few identifiable, certain aspects. However, when trying to figure out what Wiccans believe there are a few certainties.
Generally speaking, Wiccans pay respect and worship a male god and a female goddess. These figures are seen as representing all that can be, as it takes both halves to truly create and balance. The goddess is usually said to be Earth, and the god is often a horned god who isn't often named, but is her consort. Beyond this, and the application of the Wiccan Rede mentioned above, there's really nothing certain that binds Wiccans together. Many Wiccans practice in solitary, and they may add flavors of other pagan faiths (for an explanation of the word pagan and the history of the term, check it here) which makes the faith Celtic Wicca, Shamanistic Wicca, Heathen Wicca, etc. Many Wiccans, particularly women, will downplay or ignore the god in favor of making the faith more about women and women's wisdom, while others will place a paired god and goddess of a particular faith such as Odin and Frigga as their god and goddess. Some Wiccans will worship in small communities, called covens, but this is not a requirement, and in fact it's more of a way for Wiccans to find support for each other in their faith and belief.
That's really the breakdown of it. Wicca is so popular, and so widely practiced, because it allows each worshipper to make it his or her own. You could wear a pentacle or not, keep an altar or not, worship alone or with others as you will. Ironically this very ability to make the faith one's own is part of the reason that those from more rigid, centralized religions might scoff at Wicca. However, the faith is still very new as religions go and it is still growing and changing.
"Wicca: A Neo-Pagan, Earth Based Religion," by Anonymous at Religious Tolerance
"What is Wicca?" by Anonymous at Wise Geek