What is Shamanism?
By Michael Harner:

The word "shaman" in the original Tungus language (a Siberian tribe word pronounced SHAH-mahn) refers to a person who makes journeys to nonordinary reality in an altered state of consciousness. Adopting the term in the West was useful because people didn't know what it meant. Terms like "wizard," "witch," "sorcerer," and "witch doctor" have their own connotations, ambiguities, and preconceptions associated with them. Although the term is from Siberia, the practice of shamanism existed on all inhabited continents.

After years of extensive research, Mircea Eliade, in his book, Shamanism: Archaic Techniques of Ecstasy, concluded that shamanism underlays all the other spiritual traditions on the planet, and that the most distinctive feature of shamanism-but by no means the only one-was the journey to other worlds in an altered state of consciousness.

Shamans are often called "see-ers" (seers), or "people who know" in their tribal languages, because they are involved in a system of knowledge based on firsthand experience. Shamanism is not a belief system. It's based on personal experiments conducted to heal, to get information, or do other things. In fact, if shamans don't get results, they will no longer be used by people in their tribe. People ask me, "How do you know if somebody's a shaman?" I say, "It's simple. Do they journey to other worlds? And do they perform miracles?"

Over tens of thousands of years, our ancient ancestors all over the world discovered how to maximize human abilities of mind and spirit for healing and problem-solving.

The remarkable system of methods they developed is today known as "shamanism," a term that comes from a Siberian tribal word for its practitioners: "shaman" (pronounced SHAH-mahn). Shamans are a type of medicine man or woman especially distinguished by the use of journeys to hidden worlds otherwise mainly known through myth, dream, and near-death experiences. Most commonly they do this by entering an altered state of consciousness using monotonous percussion sound.

Shamans have eternally been known as "those who see in the dark."
Shamans are said to ride their drums to the worlds of spirit and back.