The predominant school of thought on hypnosis is that it is a way to access a person's subconscious mind directly. Normally, you are only aware of the thought processes in your conscious mind. You consciously think over the problems that are right in front of you, consciously choose words as you speak, consciously try to remember where you left your keys.
But in doing all these things, your conscious mind is working hand-in-hand with your subconscious mind, the unconscious part of your mind that does your "behind the scenes" thinking. Your subconscious mind accesses the vast reservoir of information that lets you solve problems, construct sentences or locate your keys. It puts together plans and ideas and runs them by your conscious mind. When a new idea comes to you out of the blue, it's because you already thought through the process unconsciously.
Your subconscious also takes care of all the stuff you do automatically. You don't actively work through the steps of breathing minute to minute -- your subconscious mind does that. You don't think through every little thing you do while driving a car -- a lot of the small stuff is thought out in your subconscious mind. Your subconscious also processes the physical information your body receives.
In short, your subconscious mind is the real brains behind the operation -- it does most of your thinking, and it decides a lot of what you do. When you're awake, your conscious mind works to evaluate a lot of these thoughts, make decisions and put certain ideas into action. It also processes new information and relays it to the subconscious mind. But when you're asleep, the conscious mind gets out of the way, and your subconscious has free reign.
Psychiatrists theorize that the deep relaxation and focusing exercises of hypnotism work to calm and subdue the conscious mind so that it takes a less active role in your thinking process. In this state, you're still aware of what's going on, but your conscious mind takes a backseat to your subconscious mind. Effectively, this allows you and the hypnotist to work directly with the subconscious. It's as if the hypnotism process pops open a control panel inside your brain.
In the next section, we'll see how this theory fits in with the characteristics of hypnosis.