Shamanism is a collection of practices done by shamans (sometimes known as witch doctors, though this is often considered derogatory), trained medicine men (and occasionally women), typically in tribal and indigenous cultures. Shamans are spiritual people with the ability to heal, work with energies, and see prophetic visions. Further, shamans have been credited with abilities such as spirit communion, weather control, divination, dream interpretation, and astral projection, among a great many other abilities.
Shamanism is based on the premise that the visible world is pervaded by invisible forces and spirits that affect the living, and is closely tied to nature. It's thought to predate all organized religion by as much as 30,000 years and have a strong influence on Ancient Hellenism and many Asian religions. Around 400 CE, the spread of Christianity virtually wiped out the shamanistic practices of many cultures.
Shamans are typically initiated as a consequence of personal psychological crisis, such as serious illness, being struck by lightning, or having a near death experience. In addition, this crisis is usually expected to have associated cultural imagery such as being transported to the spirit world, meeting a spirit guide, being devoured and emerging transformed, and being dismantled and reassembled, often with implanted amulets like magickal crystals. It's often the case that cultures view shamanistic ability as being inherited, though this isn't always the case, and occasionally initiation will include acquiring one or more familiars, the spirits of animals, healing plants, or departed shamans.
Diseases are considered to be caused by malicious spirits or witchcraft; to heal them, the shaman enters the body of the patient to find and remove the infectious spirit. This process sometimes utilizes the ingestion herbs or hallucinogens, of which many shamans have expert knowledge. Some claim to get this knowledge from the plants directly. Their work puts them in great personal risk from the spirit world, enemy shamans, and the risks of hallucinogens used for inducing alternate states of consciousness, so spells of protection are common.
Shamans are also typically said to be capable of causing harm in others, sometimes leading to suspicion of otherwise good shamans due to the sudden occurrence of illness in a tribe.