Reiki (pronounced ray key) is a generic word in Japan meaning Universal Life Energy & is described as spiritually guided life force energy.
The practice of REIKI is that of gently laying on the hands & transmitting this "Universal Life Energy" to assist the body in healing.
Because REIKI is guided by the God-consciousness, it can never do harm. It always knows what a person needs & will adjust itself to create the effect that is appropriate for them.
A person giving REIKI is called a practitioner. This is someone who has been attuned through ritual to activate the energy flow. A practitioner is not the source of energy, but only a channel or coduient for the energy.