The word Reiki is derived from the Japanese pronunciation of two Chinese characters "Rei" meaning soul, or spirit, and "Ki" meaning life force or life energy. If you read the section on Acupuncture you'll recognize the Ki as Qi or Chi. In this context they all convey the same idea of life force energy.
Reiki healinghas sometimes been compared to the "Laying on of hands" method of healing. This method, the hands method that is, is infinitely religious whereas as the Reiki method isn't religious. Although Mikao Usui did gain the ability to "heal without energy depletion" after fasting and meditating for three weeks on Mount Kurama in Japan, Reiki healing isn't dependent on religious belief to cure. More a belief in the cure and the treatment method are what's needed and that could really be said of just about any line of treatment even western medicine. If you don't trust and believe in your practitioner you're less likely to take heed of any advice or even treatment that is offered to you.
How Reiki works though isn't by using traditional methods such as sticking needles into you or giving you nasty smelling potions to drink. Reiki works by drawing energy into the practitioner, flowing through specific chakra's before finally entering the patient via the practitioner's hands.
Reiki energy is thought to be an intelligent energy knowing where to go without being directed. So once the energy has left the practitioner and entered the patient the Reiki energy will go directly to the spot that needs to be healed. If the patient is skeptical or somehow unwilling to accept treatment the Reiki energy won't work. For Reiki to work the patient has to believe 100% that the treatment method is genuine and that it will work. Not too much to ask really.