Reiki is a form of hands-on energy healing that originated in Japan. The word itself (pronounced ray-key) means Universal Life Force Energy. This energy is that same energy which surrounds us and flows through us all of the time and is recognized in many cultures as Chi, Prana, Mana, and many other names. Reiki can be channeled in a Reiki healing session so as to enhance the health of the recipient, as well as to calm the mind and soothe the spirit.
If you would be more comfortable with a more scientific explanation, think only this: in quantum physics, as with most hermetic science, everything comes down to a root level of energy and in truth there is no solid matter, but only neutrons, protons and electrons. Even human bodies, emotions and reactions are all energies: cells vibrating at different frequencies. Reiki, as such, treats those particles in vibration – an energy acting upon another energy to alter the frequency at which it vibrates.
This form of healing is passed from teacher to student through a series of initiations. In a sense, it is not unlike many Mystery Traditions in this way, in that while you may read about Reiki on the internet or in a book, there is no true substitute for a live teacher or practitioner to aid and guide you.