Health Psychology focuses on the more medical aspects of psychology and applies psychological principles to healing physical illness and medical problems. Health Psychology has grown so much in recent years that it is no longer a field made up of just Health Psychologists. Instead, clinicians, Social Psychologists, and others all conduct research on health topics. For example, a Social Psychologist may conduct studies to determine the different causes of group pressure, while a clinical psychologist may study ways to reduce stress-induced illnesses through relaxation techniques.
The APA defines Health Psychology (division 38) in the following way: "Health Psychology seeks to advance contributions of psychology to the understanding of health and illness through basic and clinical research, education, and service activities and encourages the integration of biomedical information about health and illness with current psychological knowledge." For additional information about Health Psychology you can visit the "official" health psychology website located at: http://www.health-psych.org/