The word witch doctor is one I have mainly seen used in reference to a tribal spiritual leader, shaman (as some cases may be), medicine man, and so on, as the individual group may call the spiritual head.
Otherwise, I have no useful information except to study the specific group you wish to learn about.
An African friend of mine has told me they practice healing, often with the use of herbs, and sometimes spiritual healing. They also cast spells and offer magical services for improving business, love-life etc. most charge for this or expect some form of payment. Just another witch operating under different customs and traditions is all.
The dodgier among them practice human sacrifice, which is still a problem in the modern day, many people now pierce their children's ears as they only take children who are pure- unmarked.
Solarfire and night owl15: You are both correct. But as Brysing said, it is a bit of an archaic name for tribal medicine men. In many cultures around the world, the medicine man (or woman) was healer as well as spiritual leader.
This is true even in early Europe, especially before Roman expansion. There is a reason there is so much lore about healing herbs, old wives' tales about stopping bleeding with spider webs, and the like.
The term "witch doctor" is a condescending way to not give credit to a magic user for healing ability, and to remove any credibility from a tribal doctor because of their spiritual beliefs.
As far as people in some areas charging the locals for casting spells, working magic, and placing curses, some of these are scams while some may not be; they may be someone holding on to traditions and help people while still making a living. Sometimes rumors start about things like human sacrifice to scare people away from an occult practitioner, or by the practitioner to add that feeling of taboo and dread for using their services. It's a lot like the ominous trinkets you can buy in Voodoo themed tourist shops in New Orleans. There are, of course, actual Voodoo practitioners. But people would rather buy into the dried chicken foot or alligator skull being full of some special power for $39.95 than believing anything if they saw real practices.