The words "hypnosis", or "trance", describe a normal, everyday state of focussed attention. You're in a state of trance every time you are absorbed in something to the virtual exclusion of everything else going on around you - for example when you are lost in a good book, listening to music, or just staring into space daydreaming. You also pass naturally through this hypnotic state of mind on your way in to and out of sleep every day - these are known as the hypnogogic and hypnopompic stages of sleep.
In fact, our brains are designed to naturally slip into hypnosis every 90 minutes or so as our brain activity switches from the left to the right hemisphere. This is known as the alpha state, and it allows the information that has been taken in over the past 90 minutes by the conscious, thinking mind to become absorbed and processed by the subconscious, automatic part of the mind. Think of it as the computer backing up its files.
You can use hypnosis to deliberately induce these brainwave patterns at will, in order to install, or change, the subconscious programmes that govern how you behave. A hypnotherapist doesn't "take over your mind" - quite the opposite - a hypnotist, or hypnotherapist, shows you how to focus your mind so that you enter a deliberate, voluntary state of self-hypnosis, rather than just drifting into it naturally.
In summary, hypnosis is a way to deliberately focus attention, turn it inwards and work positively with something which happens naturally. Hypnosis enhances your ability to change your automatic, subconscious behaviours, and to rapidly learn to new habits.
