It is a very ancient form of religious belief which includes human sacrifice. Usually the human is a stranger and in one instance it was Edward Woodwood, the actor.
It's a paganGanith, full of symbolism, which is mainly informed by natural cycles and phenomena, such as the cycle of the seasons and the moon. It often incorporates ritual observance of the changes in the aforementioned natural cycles and magic, or the use of the will to effect a change in the world. Wicca has been called polytheistic, since they acknowledge whole pantheons of deities, duotheistic, since many regard the pantheons as aspects of one God and one Goddess, and monotheistic since some regard even the God and Goddess as simply being two sides of the same ultimate truth.
I'm not a wiccan, so I'm not qualified to say what is true, but I think that, especially in today's world where it's mainly composed of solitary practitioners who treat the religion as something of a spiritual buffet where you can pick and choose whatever you want, that it could be almost anything anymore, depending on the practitioner.
That's a really broad question, and thus difficult to accurately answer in such a short space. If you ask something more specific you might get better answers.
Wicca is a modern, polytheistic religion with ritual generally centered upon a patron god and goddess. It stresses polarity and balance, as well as responsibility for ones own actions. Wiccans see the physical and spiritual worlds as being intimately connected. Wicca is an esoteric religion, with the real truths of it being learned through experience, rather than through instruction. The various practices of Wicca are to help people find those experiences and lessons. Wicca has no prophet and no holy books.
Those in it claim it's innocent but it is not. It is witchcraft. I'm not sure whether they worship Satan, but one teenager who got into it was seeking a way to set up an altar in her closet. Guess that shows how idolatrous it is but if you ask one who has been in it for awhile they defend this thing. I wouldn't have anything to do with it!!!
Lke any other religion, the heart of Wicca involves ones relationship with a Higher Power. Wiiccans believe in a God and a Goddess, a Father and Mother of all.
We believe God and Goddess are manifest in all things-- that life and the universe are a gift, that man is part of (not above) nature and so we see the spark of the Divine everywhere, including in ourselves.
A lot of non-Wiccans are mistaken in the belief that we worship nature; we don't. We revere the creation... but we worship the Divine Source that made it possible, that we see as inherently part of it.
We strive for balance and harmony in all things; to learn and evolve and be good people and avoid doing unnecessary harm; to take personal responsibility for ourselves and our actions.
We have our holy days, which remind of of things in life we hold as sacred and celebrate the cycles of life. We hold rituals, which are simply religious services and observances (the word ritual simply means a regularly repeated action), in which we give thanks and praise.
Other than that, like there are different denominations in Christianity, there are different traditions, or "tr ads" in Wicca, where things might vary. For example, many believe in reincarnation, but not all. Some practice magic/Witchcraft, but not all.
We have nothing against Christianity; we simply are not Christian because we do not share your views on things like the Bible being the word of a God, or Jesus/the concept of salvation, heaven, hell, devils, demons, etc... these things are just not things we believe and are not part of our path.