These days Reiki is a generic name for a number of forms of esoteric and spiritual healing methods, based (or partly based) on Usui Reiki Ryoho and related healing arts from ancient and modern Japan. Reiki is based on the idea that an unseen life force energy, chi, flows through us and is what causes us to be alive. It is used for stress reduction and relaxation, administered by 'laying on hands', and it is said to prevent disease and promote healing.
Usui Reiki Ryoho is a technique of channeling healing energy through the hands, realised by Dr. Mikao Usui in 1922. The original lore was not all brought to the West after WWII but modern expansions of Reiki Usui have proliferated. This is because Reiki is not just Usui Reiki, it is more general than that, and has been uncovered independently by spiritual healers in many cultures over many centuries.
Reiki healing energy is a special kind of healing energy - it seems to have its own consciousness and does not need to be completely guided by the practitioner. Reiki healing simply guides itself by communicating with the client's higher self and using the information received to adjust itself for the specific needs of the client. I find it is useful as a carrier for other techniques of inspection and diagnosis, and as a general purpose tool for many kinds of healing work.
When I became interested in Reiki I joined a some other people at an attunement in Usui Reiki. It became evident, from the huge amounts of powerful energy currents that I repeatedly picked up from the Reiki master, that I was sensitive in this area : I realised then that I was intuitively doing Reiki healing and it seems I am using 'old' Reiki, methods close to Usui's original style (insofar as I can establish this).