Reiki is an energy frequency that is channeled through a trained Reiki practitioner. I would describe the energy as a healing frequency akin to that of unconditional love. The practitioner has had training and has been attuned to the Reiki energy so they can act as a channel or conduit for the energy to flow through and to harnessing the energy and guiding it to facilitate healing work with clients.

People have all kinds of different experiences when receiving a Reiki treatment and each practitioner has their own way as well. Some clients can feel the energy as a physical buzzing in their body, others experience their bodies making noises, gurgling etc, some people have visualizations, smells or sounds that occur and for others dreamlike states or a deep state of relaxation describes the experience.

The Canadian Reiki Association's FAQ Page is very insightful and offers information on the history of Reiki, training and a more detailed description of Usui Reiki system.