This is a great starting point to learn about Reiki. It is described here by the University of Michigan Health Medicine department. Reiki is considered a branch of complementary medicine; alongside, acupuncture, yoga, massage, and others.
This link has a long list of personal stories submitted to the International Center for Reiki Training. Read the Reiki Articles list for wonderful testimonials to a healthier life through Reiki.
I have written about Reiki on this blog to educate both new to Reiki and those practicing Reiki. I describe my personal experiences both giving and receiving Reiki.