Hypnotherapy is simply therapy with mental imagery. It is a treatment with specific therapeutic goals and specific techniques used while the patient is in a highly focused state,* or trance. Trance is not sleep. In trance, you can communicate and use your mind to consciously control your body.
*From Olness, Karen, M.D., and Kohen, Daniel P., M.D., Hypnosis and Hypnotherapy with Children, The Gulford Press, 1996.