Witchcraft isn't the only way of learning about magic. One can approach it from many different directions depending on how you feel about magic in general. Sorcery, Alchemy, Ceremonial Magic, Witchcraft...all of these are areas where magic is practiced. Heck, you don't even have to have a name for what you do other than that you practice magic.
Witchcraft (when it is spelled with a small "w") is simply one form of magical practice, and one that is often linked with using the forces of nature, herbs, stones and crystals, etc. A person can practice witchcraft whether they belong to any specific religion or to no religion at all.
Witchcraft (when spelled with a capital "W") is a form of Pagan religion which happens to include the practice of magic as part of its belief system.
Wicca is a very specific Pagan religion with very specific beliefs and practices drawn from the pre-Christian religions of northern Europe. While Wicca also includes the practice of magic, it is different in many ways from religious Witchcraft.