There is indeed a difference between meditation (especially Buddhist) and visualization/hypnosis. As a hypnotherapist of 20 years, I have spent a great deal of time helping clients with relaxation, meditation, and hypnosis. They are all useful, but for different purposes.
Relaxation involves any method that helps persons shift from a state of sympathetic arousal to parasympathetic recuperation. Some of these rely upon external support (e.g. massage), some are dependent upon materials or places (book, movie), and still others are based upon development of self-help skills which can be utilized at any time (e.g. progressive muscle relaxation). Those experienced with meditation have much to offer those using relaxation methods (e.g. during the natural breath, tensing with an inhale, releasing with an exhale; focusing the mind on the task, so that there are attentional benefits as well as physical). Jon Kabit-Zinn's style of meditation is a great relaxation technique. Typically, relaxation methods can be used PRN (as needed), however, it certainly is best if they can be utilized on a regular basis.
Meditation involves a suspension of vigilance (to current time, place, logic, self-other distinction), and a focus on sensation (visual, auditory, kinesthetic) that is currently occuring. In other words, meditation involves a "dissociation" into the sensations present in the moment. This takes regular (daily or twice daily for about 30 minutes each session). Relaxation may or may not occur during the meditation practice. But one usualy becomes more unified in one's actions as a result of the practice (thus, expending less extraneous energy). The results are quite generalized. It seems as if meditation practice reduces "background noise" so that "signals" appear much stronger, allowing us to process our world with greater clarity of mind, emotional connection, and behavioral purpose.
Visual imagery (sometimes called the non-licensed or untrained practitioners hypnosis) usually involves a formal trance induction (if hypnosis-like) or an indirect induction (if in a story). Physical and mental relaxation is usually a necessary part of the induction phase. Thus, there is a suspension of vigilance (as in meditation), but also a focus on abstraction (past memories, optimal fantasies, metaphocial stories, etc.). This can be therapist led or practiced alone (as in self-hypnosis). It is more specific (helping with a particular problem) and can be practiced as needed.
Again, all the experiential methods (relaxation, meditation, hypnosis) are useful, but for different purposes.
I have described this in more detail in my book "Treating Dissociative Identity Disorder", published by Jossey-Bass, 1996.
I also suggest those interested in Hypnosis to BOL's discussion on Ericksonian Hypnosis, the most sophisticated approach to hypnotherapy that exists. Those wishing to get formal training in hypnosis are encouraged to join the American Society of Clinical Hypnosis (ASCH), which offers training throughout the country.