Witches fall into several categories.
1) Those who are practising something handed down through their family (this does exist).
2) Wicans, who really speaking are slightly different and follow a path of witchcraft laid out in the early twentieth century by gerald Gardner or Alex Sanders.
3) Wicca inspired witches, who follow the principles of Sanders and/or Gardiner but who have not been initiated into a coven in this tradition.

yes, many witches do work in groups that they call covens. Many also work alone. there are few hard and fast defenitions I'm afraid, for every witch there's a slightly different way of doing and being it.

It might be helpful to add that for many, Wicca or witchcraft is considered to be a path within Paganism, which also includes Druidry, Heathenry, Shamanism and the like. Generally speaking these are beliefs and practises derived from the ancient nature religions of the world. Pagans are no more good or evil than anyone else when you get down to it - its a set of philosophies, for some there's significant festivals.For some its about a very positive ethos of self discovery within a repsonsible context, for some it is intensely ecological, for some its jsut about personal growth. Some perform rituals, some don't, some explore the way in which will acts upon relaity (call it magic, call it prayer, call it positive thinknig, as you see fit.)

Many Pagan groups online spend long hours trying to come up with working defenitions of what these terms mean, I don't think we are going to achieve much on a list where a significant number of posters have no knowledge or expereince of the subject.

Unfortunatly, what you tend to see in the press and in public are the few more eccentric types who like to get in front of the cameras, and this, as with most communities, is seldom a good illustration of the whole. I would recomend that anyone genuinely interested in further information pays a visit to www.paganfederation.org