Hypnosis, as has been discussed in the previous questions and answers, deals with delivering suggestions or reworking patterns within the subconscious. Hypnotherapy uses hypnosis to go from cause back to effect. In other words, hypnosis would be the suggestion to stop a bad habit where as hypnotherapy would be tracing or discovering the original idea or reason we developed the habit.
This of course is a very simplified explanation. You may also have heard hypnotherapy being described as regression therapy.
The dictionary lists Hypnotherapy as therapy using hypnosis.