Shamanism is a practice where a practitioner reaches altered states of consciousness. Supposedly, the shaman perceives and interacts with a spirit world, and channels these energies into this world.
A shaman is a person regarded as having access to, and influence in, the world of good and bad spirits. They usually enter into a trance state during a ritual, and do divination and healing. To become a shaman, a person has to have had a near death experience (a "shamanic illness").