From http://www.asiasociety.org/arts/mongolia/shaman.html

The nomads of the vast expanse of the Mongolian steppes shared a vision of the universe and the world of human experience that was characterized by religious concepts, rituals and magical practices that came to be known as shamanism from the word "shaman."

The shaman was a kind of a priest or medium who acted as a conduit between the human world and the realm of the gods, demons, and spirits of ancestors. A person didn’t choose the profession of shaman but was selected for the job by a messenger from the spirit world. The arrival of this representative was usually announced by the chosen person’s falling seriously ill or suffering hallucinations. A shaman, called in to cure the sick person, would pronounce the patient possessed by a spirit, indicating that he or she had been chosen to be a shaman.

The chosen person was initiated by an older shaman of the same tribe into the lore of magic formulas and the songs and dances, which the shaman used to combat the supernatural sources of evil. During the initiation rite, the newly chosen shaman was given a staff. Its finial was often carved in the shape of a horse’s head, its lower end in the shape of the hoof. When the new shaman had practiced rituals for a few years, the staff was replaced with a drum. Other paraphernalia of the shaman were successively added to his arsenal.

Any sort of misfortune that came a Mongol’s way, from hunger and sickness to natural disasters, was seen as devilish mischief caused by malicious demons. Timely offerings, virtuous conduct, and strict observance of taboos would usually be enough to ensure the benevolent attitude of the ancestral spirits. However, on occasion, peace and harmony were disturbed by evil spirits. This was the moment when a shaman’s assistance was required.

The shaman did not face the forces of evil by himself. He was assisted by an army of auxiliary spirits that protected the shaman and helped him to avoid the traps set by adversaries from the spirit world. It was the number and might of these helpful spirits that ultimately determined the magic and healing powers of the shaman. In a trance induced by alcohol and the smoking of herbs, the shaman undertook his voyage to the dark realm of the demons to subdue the spirit that was the source of the problem. The shaman could also bring back to earth the spirit of someone thought to have died.  
The shaman was protected against supernatural negative forces by the magical properties of the tools of his trade. His headgear was often decorated with the antlers of deer, which gave the shaman speed and versatility. A headdress decorated with eagle feathers gave the shaman strength, while owl feathers endowed him with the power to see at night. To the shaman’s upper garments, metal objects symbolizing armor such as arrow heads, bells, and mirrors were attached. Feathers, symbolizing wings, were also attached to the sleeves of his dress. The shaman’s staff crowned with a horses head or his drum, the handle of which was also carved in the shape of a horse’s head, symbolized his mode of transportation.

When Lamaism was introduced in Mongolia, Buddhist monks undertook efforts to convince the people to abandon their shamanist beliefs in favor of Buddhist doctrine.

Differences in the two religions were marked by the ways shamans and Buddhist monks dressed. While shamans’ dress displayed all kinds of regional and even individual differences, the monks’ garb was more or less a uniform. Nevertheless, there were striking parallels between shamanistic and Lamaist rituals. Some of the same religious functions could be performed by either shaman or lama. For example, both shamans and lamas dressed in costumes with a stylized decoration suggesting a skeleton when they performed the role of Citipati, or Lords of the Charnel Grounds, in Tsam dances, and the ritual dagger of the monk, like the shaman’s staff, is crowned with a horse’s head.

Shamanism, however, was not an organized religion. Although there were "grand" and "lessor" shamans, there was no elaborate hierarchy of the kind recognized by the Buddhists. The shamans’ individualism served them well in times of persecution and suppression when they remained hard to catch. Buddhist monks, concentrated in large monastic communities, on the other hand, were more vulnerable to suppression.
In traditional Mongol society, women took the men’s place when they went off to war and had to be skilled in all aspects of animal husbandry and hunting. So it is not surprising that a fundamental equality between men and women existed and that the important functions of the shaman could be performed by men and women alike.

Shamanism in Mongolia, embedded in the nomadic life style of the people since ancient times, has managed to survive against enormous odds, including centuries of persecution by Buddhists and Stalinist efforts to eradicate this ancient tradition.

Please remember--shamans are healers first and foremost.  In some tribes, we doubled as priests, but Shamanism is only a religion unless you are talking about the Chuckchee  Tribe of Siberia.

The one thing that distinguishes us from other healers is that we work in an altered state of consciousness.  How we attain that state varies widely--the tribe in South America that Harner studied from (their name escapes me at the moment) used drugs, mainly the soul vine drink--Hawaiian shamans use sheer meditation (which is a son of a gun to do--took me about twenty years to really get the hang of it.)  Mostly, we use sonic driving--drumming, chanting, that sort of thing--to get the brain down into SSC--the Shamanic State of Consciousness--or "theta".

Virtually every culture has, or has had, shamans--the Christians in general and the Inquisition in particular--did a very efficient job of wiping it out in Europe.