Hypnosis is a state of focused concentration of the mind, heightened awareness, coupled with relaxation of the body allowing you access to the unconscious mind.
The unconscious mind stores information and is also the place of your creativity and intuition. In this state you are open and receptive to positive suggestions and imagery which create desired future outcomes.
The conscious mind is always aware and you are in complete charge, with the ability to stop at any time. The therapist is essentially a facilitator or guide. It can be used for many things including past life regression.