Hypnotherapy utilises the altered state of consciousness of hypnosis to enable clients to become open to learning new positive ways of thinking, feeling and behaving to replace unwanted thoughts, feelings and ways of behaving. This learning takes place at the deep subconscious level and so as your mind accepts these changes permanent, positive change can follow.
A client may be guided into hypnosis by the therapist's voice, simply using words. The hypnotherapist may guide the client into a deep hypnotic state or they may use "conversational hypnosis" which is a more informal approach. Conversational hypnosis as the name suggests will seem like just having a conversation. Many therapists will work in both ways, depending on which works best for the client.
Anyone can be hypnotised if they want to be. Equally people cannot be hypnotised if they don't want to be. The client remains in control throughout.