hypnosis is a process that uses particular states of consciousness. It can include self-hypnosis or allowing oneself to be hypnotized by others. It may be done out of curiosity, for experimentation, to help with habit change, to achieve goals, or for therapeutic reasons, that is, to alleviate mental or physical symptoms.
Hypnotherapy is therapy using hypnosis. It can include allowing oneself to be hypnotized by a hypnotherapist, and it can include receiving training in self-hypnosis.
I have been facing severe, chronic pain, largely untreatable, for several years. I have chosen to do this without Western medical treatment and almost entirely without pain relievers.
I've followed a path of meditation and healthy exercise (qigong and yoga, mostly), rather than hypnosis. I believe they are more powerful. But I would use hypnosis before I would use pain relievers or Western treatments.