Hypnotherapy can be variously described as "a talking therapy" or as a form of psychotherapy or, alternatively, as a psychological intervention.
But whichever description you choose, it is a therapeutic technique which uses the hypnotic state to (1) identify the causes or origins of personal problems (2) provides means of reducing or overcoming the problem or problems.
In broad terms, this is achieved by the therapist helping the client to enter a hypnotic state, often by means of progressive relaxation. The therapist induces this state by using the voice, with different suggestions, to gently lead the client into relaxation.
As for the hypnotic state, that is simply an "altered level of awareness" which most people experience as pleasantly relaxing, similar to daydreaming.
The conscious mind then becomes calmer and less active, allowing the subconscious part of the mind to take precedence. This is the part of the mind which contains all the resources necessary to make beneficial and effective changes in thoughts, beliefs and behaviours which are no longer helpful or relevant. This is achieved by means of suggestions and negotiation presented to the client in hypnosis.
The subconscious mind, which tends to accept suggestion much more readily than the conscious mind, will then incorporate the positive suggestions in to the client's belief system, allowing them to make the desired behavioural changes
Hypnosis however is emphatically not mind control and the subconscious mind will not accept any suggestions which conflict with the client's moral code or which they would not accept in their normal waking state.
Clients usually come out of hypnosis feeling very calm and relaxed, as if they have awoken from a deep refreshing sleep.
Some problems may be dealt with in one session only (such as smoking cessation) while others may require a course of treatment. The method and duration of treatment is usually agreed in the initial consultation.
Hypnosis is also sometimes called a state of "heightened suggestibility" which is a reasonable description, but only part of the story. What is known is that hypnosis is an entirely natural and usually hugely beneficial state, permitting greater access to the unconscious part of the brain.