The word Pagan is really a generic term that covers a lot of very diverse religious groups. Many people use the term to mean any religion that is not Christian, Jewish, or Muslim. Some religious groups that are not Christian, Jewish, or Muslim contest whether they are Pagan or not.

Pagans include Druids, Wiccans, and many other groups. Some of these religions are new, some are attempts to recreate old religions, and some claim to be an unbroken continuation of ancient faiths.

Witches are people who practice magick in some form. Witchcraft is a practice that can be incorporated into any religious framework, or practiced without any specific religious overtones. Since the term Witch has been used as an insult for so long, many who practice magick choose to avoid the term Witch. It is usually polite to only use the label Witch for those people who choose to use it for themselves.  There are many who practice forms of magick that are not considered to be witchcraft, and who do not call themselves Witches.

Wicca is a religious system that draws its philosophy from pre-Christian as well as more modern sources. Many Wiccan practitioners incorporate Witchcraft as a central element of their spiritual practice, although there are some Wiccans who do not practice magick and who do not consider themselves to be Witches. 

The important thing to remember is that not all magickal practitioners are Witches, not all Witches are Wiccans, not all Witches practice Witchcraft as an integral part of their spirituality, and not all Pagans are either Witches or Wiccans. While there is certainly overlap in the terms they are not interchangeable.