Wicca is a modern religion which is based in part on the pre-Christian religions of Northern Europe. Wiccans are polytheists in that they believe that the Divine manifests as both God and Goddess, who may be known by many names. The practice of magic is part of Wicca; although, a small part. Thus Wiccans are considered to be Witches...but not all Witches are Wiccans.
You can find my post about the core beliefs of Wicca at http://www.spellsofmagic.com/read_post.html?post=248610
There is an excellent FAQ about what Wicca is that can be found at http://wicca.cnbeyer.com/wicca101/index.shtml