Historically speaking, witches were people who were hunted down by the church and found guilty of doing witchcraft.
Besides for displaying psychic abilities, a person could find herself on a burning stake for being a social reject, for angering the wrong people, for following different spiritual traditions, for willingly or unwillingly evoking fear in others, or for simply being a woman with a lowly social status.
(It's a familiar situation: by playing to the intense, subconscious fears of the masses the few powerful people could pretty much doom anyone to death.)
Every village had a wise person who was in possession of some knowledge passed down through the generations, and to whom people turned to help of semi-spiritual nature. In these times being a witch was not a voluntary declaration of identity, but a judgment from others.
A modern, self-declared witch is someone who is pursuing the path of the psychic through the few tradition-remnants of these ages.