According to the American Cancer Society's website:

Reiki is said to promote relaxation, decrease stress and anxiety, and increase a person's general sense of well-being. The Reiki practitioner delivers the therapy through his or her hands, with the goal of raising the amount of universal life energy (called qi, ch'i, or ki) in and around the client. Reiki supporters claim that when the energy paths of the body are blocked or disturbed, the result can be illness, weakness, and pain. Reiki practitioners intend to strengthen the flow of energy, which they say will decrease pain, ease muscle tension, speed healing, improve sleep, and generally enhance the body's ability to heal itself.
During a Reiki session, the practitioner places his or her hands in 12 to 15 positions on or above parts of the patient's clothed body. The hands are intended to be a conduit for universal life energy, balancing energy within and around the body. The hands are held in place for approximately 2 to 5 minutes in each position. A Reiki session usually lasts about an hour. Some practitioners say that they achieve the best results when patients have 3 Reiki sessions within a relatively short time, take a break, and then repeat the process. There are 3 levels of Reiki practice. A Reiki I practitioner can offer hands-on sessions; a Reiki II practitioner can offer hands-on or distant Reiki; and a Reiki master can offer hands-on Reiki, distant Reiki, and Reiki instruction. The second degree Reiki practitioners and Reiki masters believe that they can send healing universal life energy over any distance, similar to claims by qigong masters who practice traditional Chinese healing concepts.

The basis for modern-day Reiki practice may have started in Tibet more than 2,500 years ago. Reiki was rediscovered in the early 1900s by a Japanese man named Dr. Mikao Usui. During a lengthy period of travel and research, Dr. Usui found ancient texts that described Reiki and its power to heal by using the energy that flows through all living things.