Simply put, hypnosis is a natural state of mind in which the conscious mind becomes quiet, and the subconscious mind becomes more open to imagery, suggestions, and change. It's similar to that in-between state you experience when you're almost ready to fall off to sleep, but you're not yet asleep, nor are you fully awake anymore. Some say it feels like meditation, others compare it to being absorbed in a daydream.
Everyone experiences hypnosis a little differently. Some people feel a sense of heaviness in their bodies; others feel lightness. A tingling sensation in hands or feet is quite common, and one's sense of time is often distorted. Once in a while, someone tells me they became very aware of a particular color. Almost always, my clients report feeling very relaxed and peaceful.