Reiki session is a Japanese energy healing technique. It’s called palm healing through which the "universal energy" is channeled by the practitioner and transferred through the palm to the patient. 

It's originally formulated in Japan by Dr. Usui, although it's the oldest energy technique on earth but he had the power to channel it and set it up into a useful technique that can be thought to generations to come. 

It's a pure energy work that does not consist of using any other tool, although using crystals or breath work with it can help but the technique itself is sufficient to deliver the perfect outcome.