A personal friend asked a great question after I told him about The Behavioral Medicine Report: What is Behavioral Medicine? Behavioral Medicine and the related Health Psychology are sub-specialties in psychology.
Behavioral medicine is an interdisciplinary approach (i.e., utilizes knowledge from different fields of healthcare) to the development and integration of behavioral and biomedical information to the prevention, diagnosis, treatment, and rehabilitation of physical and psychological disorders. The behavioral medicine part of my degree led me to study, for example, psychology, physiology, neurology, biology, immunology, and psychophysiology. Persons trained in behavioral medicine typically do not study other fields at a micro-level, else few practicing Behavioral Medicine Psychologists would exist because they would never graduate; rather, they study other disciplines at the macro-level.
Health Psychology is a separate but related field. Think of Health Psychology as an umbrella for all psychologists working in health and illness, including Behavioral Medicine. Health Psychology is the educational, scientific, professional, and psychological contributions to promote and maintain health, to prevent and treat illness, and to identify etiological (origins) and diagnostic correlates of health and illness. Clinical Health Psychology is the application of these principals in a clinical setting with patients instead of research participants. While behavioral medicine may deal in health and medical illness (non-mental illness), Health Psychology almost always relates to it.
Finally, Health/Behavioral Medicine Psychologists often are trained in a biopsychosocial approach to assessment and treatment. This perspective acknowledges the role and interplay of biological, psychological, and social factors in the way people feel, think, act, and how this affects their health and illness. The biopsychosocial model is also a good reason why Health/Behavioral Medicine Psychologists are versed in broad range of treatment approaches. My experience has been that patients prefer this type of relationship with their healthcare provider once they experience the biopsychosocial treatment model.