Reiki...broken into two parts of the word: Rei means universal and ki is life energy... is a Japanese relaxation technique. It involves laying hands on someone (or hovering the hands above the body 3-4 inches in states where the practitioner is required to be either a doctor, nurse or licensed massage therapist in order to physically touch anyone) and channeling the universal life energy into the client. The Reiki practitioner is certified by a Reiki Master (higher level practitioner) to channel that universal energy.
Everyone is born with this energy and it exists all around us, all the time. Some say it is energy coming directly from God, while others speculate is comes from the actual universe in which we live. When the energy is channeled into someone, that person becomes very relaxed and most often, if the receiving person has a physical pain or ailment, that pain or ailment either completely disappears from an unknown amount of time or at least improves.
The general idea of "healing" comes from the fact that the human body possesses 7 energy centers, called chakras. These chakras becomes clogged when the person is bombarded with negative energy (this can come from negative thoughts or negative input, such as horrible news on tv or a negative person feeding them negative information). The Reiki practitioner utilized the energy to clear and balance these chakras. Once they are clear, the human body will proceed to fix itself in any way that it can. It's not instantaneous, although the pain part, and sometimes stiffness, does goes away immediately in a lot of the cases I've experienced. Several sessions are usually required to really clear some "ailments" (which can be mental, emotional or physical). Mind you, it won't fix a broken bone or a serious illness but I've seen it temporarily relieve arthritic, lack of mobility, in hands. It amazes me every time I see what it does.
The person being "healed" can either sit in a chair or lay on a massage table. I find the table to be the best method as I can more easily access the various chakas... but I have done a whole lot of sessions with the person sitting in a regular chair.
So, there you have it. There is a web site that explains the whole concept:
http://www.reiki.org/FAQ/WhatIsReiki.html It's really quite good. I have only acquired my level II but this summer, I expect to get my level III, which is Master in which case I'll be able to "attune" others so this practice can spread.
Thanks so much for asking about this.