The induction and use of trance states has been around for thousands of years with mentions being made in the surviving artefacts of ancient civilisations form Africa to Australia and South America. However it was only in 1843 that James Braid gave the term 'Hypnosis' (more accurately neuro-hypnotism) which was given to it from the Greek God of sleep - Hypnos. This followed on from the experiments into trance states by Anton Mesmer and his followers into the theory and practice of magnetism; thus modern hypnotism was created.
In the 20th century much scientific research was done by Freud, Weitzenhoffer, Hilgard, Rossi and, of course, Erickson which has led to the its current understanding as an altered states of awareness and schools of thought/ practice in hypnotherapy such as Traditional, semi-traditional and Ericksonian (Naturalistic).
The hypnotherapist is a skilled guide to the process who enables the subject to relax as deeply as possible and listen as the subject in hypnosis mentally drifts in a safe, comfortable, state of altered awareness where new learning experiences and suggestions can easily be assimilated and accepted.
If the subject doesn't choose to go into hypnosis, or they don't feel comfortable or safe then a trance state will not be induced; it is a partnership agreement between the hypnotherapist and the client.
Hypnotherapy is therefore simply the use of hypnosis in a therapeutic setting to bring about beneficial, wanted changes.
Hypnosis is not a state of sleep as in the sleep we have at night, but more like the states of going to sleep or awakening from sleep and day-dreaming. An individual in hypnosis is aware of their surroundings at all times and can hear everything that is said by the hypnotherapist. All hypnosis is effectively 'Self-Hypnosis' that is guided by the hypnotherapist.
Hypnosis is a naturally occurring state that spontaneously occurs many times during our waking day. It is a state in which the nervous system switches from an anxious to more relaxed state of operation and the unconscious mind is more open to new learning or suggestions that are made to help change old, restricting, habits and beliefs to newer, desired ones without interference from the conscious mind's 'editing' process.
The most important factor about hypnosis is that the subject is always in control, he/she can come out of hypnosis at any time they want, the hypnotherapist has no power over them and can only create guided experiences or make suggestions. The subject in hypnosis cannot be made to do anything/behave in a way that is in conflict with their own moral code; neither can they 'get stuck' in hypnosis, they would awaken naturally the same way that you do from sleep.
When the subject is in hypnosis - be it a light, medium or deep trance-like state everything becomes slower, breathing, heart-rate and movements are reduced or virtually absent as everything becomes a 'bit too much of an effort'
It is very different to the perception of hypnosis and post-hypnotic suggestion that is portrayed by stage hypnotists - exhibitionism, role playing and peer pressure have more to do with that that the near motionlessness of therapeutic, clinical hypnosis.