Reiki is an energy balancing system and is based on the channeling of a form of energy, or a subtl energy. This energy can be referred to as 'chi', which you may have heard of. When you visit and acupuncturist they use needled to encourage energy flow through a series of meridians or energy channels that run through the length of your body; that energy is called 'chi'. 

 

You may have heard of a Japanese massage technique called Shiatsu, which uses finger pressure on acupuncture points to achieve a similar effect; the Japanese adopted the Chinese view of the body's energy system. You will probably have heard of Tai Chi (or maybe Chi Kung/Qigong), a graceful system of exercises that are designed to build up, or cultivate, your personal reserves of this energy, and the graceful movements that you make serve to circulate this energy smoothly throughout your body, breaking down any blockages and bringing things into balance on all levels. The energy is seen as animating all living things.

When we move from China to India, the same energy is referred to as 'Prana', and breathing exercises and yoga techniques have been developed to again bring your energy system into balance. In India they do not think in terms of meridians, but of chakras: energy centres running the length of your body from the crown of your head to the base of your spine, seven main ones and further subsidiary ones in other locations.

The intention with all these energy techniques is that if you can harmonise your energy system, you are putting your body in the best possible position to heal itself on all levels: physical, mental, emotional and spiritual. In fact, the Chinese view is that chi not only flows through your body, but surrounds and engulfs you, too. For example, Feng Shui has been developed as a way of arranging your living environment to allow for the smooth flow of chi around you, eliminating areas where stagnant chi might accumulate, for example.
Reiki deals with this same energy, to an extent.