Reiki is one of the more widely known forms of energy healing. Energy Healing involves direct application of Chi for the purpose of strengthening the clients energy system (aura). Chi is the term used by the Chinese mystics and martial artists for the underlying force the Universe is made of. Mystics in all cultures have talked about the physical universe being made of an underlying form of something, much as modern physics research is now coming to understand the Universe is made of energy which is subject to (or affected by) thought. Just as modern physics says this energy is affected by thought the mystics also say this underlying form is affected by thought, going so far as to claim we create our own reality from our thinking and the thoughts we share between each of us every day.
Stop and reflect for a moment. Imagine the implications of the universe around us made from energy which can be shaped and manipulated by thoughts. This is the implication of both Quantum Physics and ancient Metaphysics. Might this make some diseases easier to explain, especially those which have no apparent physical cause? Might this make miraculous cures easier to explain? For example there was research study done a couple years ago showing that prayer improves the health of those who are prayed for, the focus of intent in prayer sends thoughts out in the form of "I wish such-and-so to happen", so if the world is energy subject to the power of thought then prayers must work (depending on how clearly and carefully you hold and elaborate on the prayers or thoughts). Remember this while reading here about Reiki.
Reiki is very easily learned, very simple to use, and beneficial for all. It is one of many forms of healing through the use of the natural forces which were given the name Chi by ancient chinese mystics. Some forms of healing using Chi energy forces are Chi Gong, Pranic Healing, Chelation (as taught by Brabara Brennan and Rosalyn Bruyere), and Polarity Balancing. All (apparently) use the same energies, with the difference being techniques of application and an energy quality commonly known as vibration. (This term vibration is widely used in the "New Age" community to refer to the range in experience between the dense or low vibration of the physical world to the high vibration of the highest spiritual expression; while the term is inadequate to truly describe the differences, and while Reality is probably not a simple bipolar scale of experience, the term is descriptive of how the energy feels). I'm reluctant to definitively say Reiki uses Chi because I am not an expert on what the Chinese mean by this word. To my eye the force referred to with the name Chi is the same as Prana, Orgone, Odic, they are all names for the same force.
Since the word Reiki means Universal Life Force Energy, it (strictly speaking) refers to the energy and not the specific techniques taught by Reiki Masters. The techniques are more properly called the Usui System of Natural Healing after the founder of the system, Dr. Mikao Usui. However as Reiki is the common name, and a fine name at that, this is the name we will use here.

A Reiki healing is very simply performed. The practitioner places his or her hands upon the person to be healed with the intent for healing to occur, and then the energy begins flowing. The Reiki energy is smart since the Universe is a very smart place indeed. The energy knows where to go, and what to do once it gets there, or else is being directed by a higher intelligence. The energy manages its own flow to and within the recipient. It draws through the healer exactly that amount of energy which the recipient needs. All this happens without direct conscious intervention by the healer. The healers job is to get out of the way, to keep the healing space open, and to watch/listen for signs of what to do next.

Just for today
do not worry.
Just for today
do not anger.
Honor your parents, teachers
and elders.
Earn your living honestly.
Show gratitude
to everything.

Dr. Mikao Usui

Author's Bio: 

Site healing member and member of Ethical Priciples of Reiki organization. Recently moved to Master level II.