Ask a Consulting Hypnotist or a Hypnotherapist this question and you will get many answers at many levels of detail and descriptions. It can be generally agreed that the experience of hypnosis is that of a state of light sleep wherein you are aware of what is occurring in your immediate presence. Hypnosis is derived from the Greek work Hypnos which means sleep.
This light sleep is referred to as a trance, and is not actually sleep because you do not lose conscious awareness unless it is a part of the session or suggested to do so. In this trance, your conscious, awake state, has taken a back seat to your subconscious. While you can still talk and listen to the Hypnotist, it is not your primary focus. It is through the skill and experience of the Hypnotist that you get to the trance state and to provide suggestions while in that trance state to accomplish your goal. The goal being weight loss, smoking cessation, or a past life recall, etc. In doing this, your subconscious is learning something or accepting a new way of performing or approaching daily life activities that will help you with your issue of concern. The subconscious is always awake and functioning, you are simply not aware of it. When you fall asleep, you do not stop breathing, perspiring, or digesting food because your subconscious is controlling your bodies every function and needs to support life. When you are awake and grab a pencil off the desk, you do not consciously think of every muscle movement and timing to accomplish that task, but it is your subconscious that does that as a reflex action to your conscious desires.
Everyone is capable of being hypnotized, if they desire, as well no one can be hypnotized against their will and made to do something they would not do at their own discretion in their everyday life. We enter trances many times a day while driving down a boring road or reacting to marketing strategies using subliminal programming. Subliminal programming is giving suggestions to people through their subconscious without the conscious really being aware of the suggestion. Many of us then react to that suggestion in our conscious decision making without realizing we were persuaded to make that particular choice.
So the hypnosis experience is an effort to break a habit or improve our performance by going directly to the subconscious and entering a suggestion that we are willing to accept for our benefit.