A witch can be used to refer to a person who practices witchcraft or magic, and may also refer to a Wiccan, a person who practices the religion Wicca. The word is now applied almost exclusively to women, though in earlier English it applied to men as well. Most people would now call male witches sorcerers, wizards, or warlocks, although Wiccans continue to use the term witch for all who practice witchcraft.
The belief in witches has always existed in nearly every region of the world, including Europe, Asia, Africa and the Americas. In Western culture, the concept of a witch has existed since at least the days of the ancient Greeks, as witches figure prominently in many Greek tragedies.
(A good list of references to witches in ancient history would be good here.)
American and European Witches
During the middle ages and up to about the mid-19th century, witches were universally associated with evil, under the belief that the witch's magical powers were granted by Satan in exchange for the witch's soul. Many outrageous claims were made about the powers of witches, which include the ability to fly, to transform oneself or others into animals or other shapes, and to curse one's enemies.
It was extremely dangerous to be accused of being a "witch", since a common punishment was to be burnt at the stake. Both in North America and in Europe, thousands of people (mostly women), were put to death as witches at various points in history. Some of the worst witchhunts were in Germany, though there are documented cases of torture and murder in the name of stopping witchcraft in nearly every European country.
Most people who were killed as witches were probably hapless midwives, herbalists, widows, spinsters, social outcasts, or victims of revenge seekers. For example, some researchers wholly attribute the Salem witch trials in 1692 to rivalries between opposing political forces in Salem, Massachusetts. See the extensive discussion under witchhunts.
Witches in Modern Culture
In modern days, few people believe in witches that curse enemies, change shapes, or can fly. However, since the last last half of the 1800's Neopagans (mostly Wiccans a subset of Neopagans), have called themselves witches and while most of western culture continues to assign negative connotations to the word, to a Wiccan, it is not a derogatory term, nor does it have anything to do with Satanism. In fact, many Wiccans wish to reclaim the term "witch" and make it positive. The term "white witch" is somtimes used to refer to an exclusively positive meaning of the word.
Today, witches are iconically associated with Halloween, though Wiccans actually celebrate Samhain. Both dates are the same, and are at least metaphorically similar in meaning. This is not coincidence. Christianity had a basic contempt for the supernatural overtones of the festival. The association between "witches" and Halloween most certainly came from vilification of practicioners of the Celtic celebration of the last harvest.
Witches also appear as villians in many 19th- and 20th-century fairy tales, folk tales and children's stories, such as "Snow White and the Seven Dwarfs", "Hansel and Gretel", "Sleeeping Beauty", and many other stories recorded by the Brothers Grimm. Such folktales typically portray witches as either remarkably ugly hags or remarkably beautiful young women. In the classic story The Wonderful Wizard of Oz by L. Frank Baum the villan is a bad witch but two good witches play important roles as well.
Witches have come into the mainstream in the last decade as well as common pop-culture figures. Teenage and young adult witches have been the focus or appeared in the movies "The Craft", "Practical Magic" and "Blair Witch Project 2" (the sequel to The Blair Witch Project), as well as in the television programs "Buffy the Vampire Slayer", "Charmed", "Sabrina the Teenage Witch", and episodes of "The X-Files". Such neo-Gothic portrayals bear little relationship to Wicca, or even a Christian view of witches for that matter. In almost all cases witches portrayed in movies and TV shows today are attractive women who have supernatural powers.
See also: Malleus Maleficarum.