From Ian Lawton's Hypnotherapy page:
What is hypnosis?
Hypnosis happens naturally whenever a person is inclined to accept outside suggestions. Hypnosis makes memories more real and ideas clearer. It can act to help you heal more rapidly, be more creative, and change for the better.
How does hypnosis work?
Hypnosis happens automatically anytime a person becomes deeply relaxed or highly focused. It happens when a person daydreams, while driving, reading a book, or watching a movie. Through the use of carefully worded suggestions, a professional hypnotherapist can prompt and fine tune hypnosis in clients and help them to make changes.
From Martin Twycross' hypno therapies page:
Hypnotherapy is used for therapeutic change whilst under the state of hypnosis. It is a completely safe and natural form of therapy.
Every individual has conscious and subconscious parts of their mind. The subconscious stores emotions and events from our lives. Everything we’ve ever seen, learnt or experienced is stored in this part of the mind.
The subconscious is also there for our protection and will warn us of dangerous situations should they arise. It is the subconscious part of the mind that is responsible for sending the messages around the body such as anxiety and adrenalin to warn us we are in danger. The subconscious is always working, it never sleeps and is always absorbing information. It is the subconscious that gives us our dreams.
Our conscious minds, in comparison, are used for our everyday activities to solve problems, use logic and reason to decide what we are going to do. Normally these two parts of the mind work independently from one another. Our conscious thoughts determine the here and now, what we want to do and how we're going to do it.
The subconscious mind is working on what it knows to be right from past learning and past experience. Sometimes things that have been stored within our subconscious create problems and conflict. This can create a distressing situation for the individual not knowing why they feel unable to do things. Traumas, habits and emotional distress can all build up and be trapped in subconscious part of the mind.
This is why using willpower can be so difficult because you have conscious thoughts and desires battling against subconscious instilled processes. And when there is a conflict between logic (consious) and imagination (subconscious) the imagination will always win!
This is where hypnotherapy comes in. Under the state of hypnosis the subconscious mind is receptive to new suggestions and the problems can be treated at source. The old habits and processes can be overridden.