Reiki is an energy healing technique. Energy healing is a holistic practice that activates the body's subtle energy systems to remove blocks. By breaking through these energetic blocks, the body's innate ability to heal itself is stimulated. See below for information on Energy Medicine. 
It can help with anxiety and stress, pain, disease, and any number of challenging things we experience as humans. It works with our physical, emotional and mental aspects and can give powerful assistance to our health and well being.
Ancient cultures used different modalities (acupuncture, massage, traditional Chinese medicine) to stimulate the body's natural ability to heal and they all recognized the power of energy.
The word Reiki is made of two Japanese words - Rei, which means 'higher power' and Ki which is 'life force energy'. So Reiki actually means 'spiritually guided life force energy'.
Go here for more information about Reiki
Spiritual Wisdom
Life Force Energy
A treatment may feel like a wonderful glowing radiance that flows through and around you. Each person's experience is different. Reiki is known to treat the whole person including body, emotions, mind and spirit, creating many beneficial effects such as relaxation and feelings of peace, security and well being, and in many cases, aids healing in the body for physical ailments.
It's important to note that energy healing is a complementary therapy that should not exclude any Western medicine you may be taking.
I am a qualified Reiki I and II channel (for 34 years) and can administer both one-on-one and distance treatments.
Reiki was passed down to me in 1987 (I) and 1988 (II) by Reverend Beth Gray, Reiki Master, and is the original Usui Reiki.

What is Energy Medicine?
Quantum physics teaches us there is no difference between energy and matter. All systems in the human being, from the atomic to the molecular level, are constantly in motion-creating resonance. This resonance is important to understanding how subtle energy directs and maintains health and wellness in the human being. Energy medicine (or healing), whether human touch or device-based, is the use of known subtle energy fields to therapeutically assess and treat energetic imbalances, bringing the body’s systems back to homeostasis (balance). The future of this depends on the ability of allopathic medicine to merge physics with biochemistry. Technology needs to be expanded to include the existence of the human biofield (or human energy field) to better understand that disturbances in the coherence of energy patterns are indications of disease and aging. Future perspectives include understanding cellular voltage potentials and how they relate to health and wellness, understanding the overlap between the endocrine and chakra systems, and understanding how energy medicine therapeutically enhances psychoneuroimmunology (mind–body) medicine.
Energy medicine has been defined as a branch of integrative medicine that studies the science of therapeutic applications of subtle energies. For centuries, allopathic or Western medicine has investigated the body’s internal systems—from organs, tissues, and cells to the current understanding of hormones and peptides. The human organism has many aspects that are not physical—aspects that generate and absorb massive amounts of information. Physiology interacts with its environment via ambient fields such as light, sound, electricity, magnetism, and with all other living organisms to generate massive amounts of information in the form of energy fields. These energy fields are involved in the therapeutic effects of pulsed electromagnetic field on immune function and tissue regeneration, on organ-associated frequencies instrumental in the endocrine/chakra systems, and on the regulatory mechanisms of neurotransmitter conversion of external fields into chemical or electrical energy involved in mind–body function known as psychoneuroimmunology. The subtle energies involved in these systems exhibit the internal and external aspects of the human being described as the human biofield or human energy field.

Sound Healing is also a form of energy healing, see Sound Healing in the menu above.