Hypnosis is a state of intense concentration or focused attention marked by increased suggestibility. The hypnotic trance state is created by way of an "induction". An induction is a way get the conscious mind to take a back seat and allow access to the unconscious mind.
There are many forms of hypnotic inductions that can lead to the hypnotic trance state. They range from the progressive relaxation technique that can take minutes to hours, to "instant" inductions that can have a person open to verbal suggestions in split second.
When used as a form of therapy, hypnosis is often induced using one of the relaxation techniques, but more rapid inductions also have a place in the office.
For street and stage hypnosis, the more rapid inductions are used almost exclusively.
At their core, all inductions have one thing in common, they must create a trance-like state by bypassing the critical faculty of the conscious mind allowing hypnotic suggestions a clear path to the subconscious.
For more analytical subjects, confusion techniques and indirect suggestions are often the easiest way to accomplish hypnosis. For the less analytical a wider variety of inductions and more direct suggestions are called for.
When hypnosis is used as a form or therapy, especially in conjunction with other behavioral therapy techniques, deep trance is not necessarily required. Things like pain control for physical chronic pain can be accomplished at remarkably light levels of trance.
Of course, on stage, a deep level of trance is often a plus as it enhances the effectiveness of hypnosis as an entertainment tool.