Reiki is a Japanese form of alternative medicine called energy healing. Reiki practitioners use a technique called palm healing or hands-on healing through which a "universal energy" is said to be transferred through the palms of the practitioner to the patient in order to encourage emotional or physical healing.
The hands-on technique as well as distance reikis promotes stress reduction, relaxation, and healing. There are many lineages out there, and the one I am attuned to is Usui Reiki. The most effective lineages will be able to trace an unbroken line to the founder - in my case, Mikao Usui. There is a belief that Reiki energy has always existed and that it has been brought to Earth and harnessed by many people throughout human existence.
The system taught by masters of Usui Shiki Ryoho emphasizes the value of self-treatment as the core of the practice, encourages the treatment of family and friends, acknowledges this as a life-long practice through which the depth of the gifts of healing and growth are manifested.