Basically hypnosis is a state of consciousness that allows a person to be open to positive suggestions. We move through different states of consciousness on a daily basis. Think about when you have been fully focused on a task at hand compared to when you are day dreaming. The hypnotic state has been described as being similar meditative state.
There are hypnotic techniques that can lead a person to a state that resembles sleep and there are other techniques that allow a waking hypnosis, this can be very good for sports people, performers and sales people trying to get into the "zone".
Being in a hypnotic state helps to focus a person onto a single idea/belief that they want to change. The therapist gives direct positive suggestions to the person's unconscious mind. The person in a hypnotic state has complete control over the suggestion they take and not take.