A witch is considered to be a person having a great power and authority and this power is utilized or harming others. Usually a witch is a woman who rides about at night on a broomstick.
During earlier times people lived in ignorance and superstition thus the witch craft firmly rooted itself on the hearts of people.
Around 17th century both Roman and Catholics protestant began a witch hunt, in England professional witch finders were appointed to find these witches and hundreds of people were accused of witch craft. A male witch is known as wizard or warlock.
During early Christian times witches used to sell their souls to devil, a pagan God in return they used to get supernatural powers. Men cannot understand certain phenomena. Thus due to ignorance they believed in witch craft.
Below Witches info from Wikipedia (Read More):-
A witch (from Old English wicce f. / wicca m., also compare hag) is a practitioner of witchcraft.
Witchcraft, in various historical, anthropological, religious and mythological contexts, is the use of certain kinds of supernatural or magical powers. Witchcraft can refer to the use of such powers in order to inflict harm or damage upon members of a community or their property. Other uses of the term distinguish between bad witchcraft and good witchcraft, the latter involving the use of these powers to heal someone from bad witchcraft. The concept of witchcraft is normally treated as a cultural ideology, a means of explaining human misfortune by blaming it either on a supernatural entity or a known person in the community.
http://en.wikipedia.org/wiki/Witches