The following are selections from various pages. The URL's are listed after each comment.

What is Hypnosis?
The subconscious mind controls our emotions, our self-esteem, our beliefs, our immune systems, and our bodies. It is filled with awesome mysteries and unguessed powers. Hypnosis is the primary method used to contact the subconscious mind.

The most popular traditional view of hypnosis is a sleep-like state induced by a procedure of some kind by an operator and in which certain special behaviors seem to result; particularly extreme responsiveness to suggestions made during the hypnotic proce ss, including physiological responses, and where anomalies of the experience of volition and memory are consistently reported by subjects.
(http://www.utu.fi/~jounsmed/asc/hyp/faq0.html and http://www.hollys.com/alchemy/qanda.htm)

Can anyone by hypnotized?
Using standardized induction scripts and classical induction techniques, some people are found to be markedly more hypnotizable than others. Aside from a requisite minimum intelligence for language and capacity to follow instructions, there are some other stable characteristics that seem to relate to hypnotizability, though they do not appear to relate directly to anything that we ordinarily consider personality traits (such as the stereotype of gullibility and so on).

An exceptionally skillful operator can individualize their approach and thereby reduce the number of 'unhypnotizable' or 'resistant' subjects quite a bit, but there are still some people that respond much more easily than others to hypnotic suggestion, es pecially with regard to 'deep trance' phenomena. This responsiveness appears to show high test-retest reliability, even after many years.

There are 12 standard tests in the SHSS (Stanford Hypnotic Susceptibility Scale) which measure how well a subject conforms to the behavior of a classically hypnotized person. By these scales, about 5% of people are classically unhypnotizable, most people show moderate scores, and about 10% are hypnotizable to extreme depths and show the classical deep trance phenomena such as somnambulism, visual and auditory hallucinations, and ability to remain deeply in hypnosis with eyes open.
(http://www.utu.fi/~jounsmed/asc/hyp/faq0.html)

Under Hypnosis can I be forced to act against my will?
Individuals can probably be influenced under a situation of contrived hypnotic imagery to do things that would ordinarily be considered very unusual, and to do them at unusual times and places. But there are clearly 'ecological' limits to this as well.

For example, most studies have sugggested that the individual can and does reject suggestions of some types, in some way, both during hypnosis, and in the form of post-hypnotic suggestions, and is not being coerced directly under hypnosis to act against t heir 'will' in any meaningful sense.
(http://www.utu.fi/~jounsmed/asc/hyp/faq0.html)