Social medicine aims to understand how social and economic conditions impact health, disease and the practice of medicine and then try to work out how we can foster conditions to lead to a healthier society. Social Medicine focuses on the wider determinants of health or 'upstream' factors and include housing, education, income, poverty, transport, health care organization and environmental and genetic influences. By targeting these factors we can prevent illness from happening and so save costs and lives.
Social medicine uses social science and humanities research to improve the practice of medicine, the delivery of treatment and development of healthcare policies. Public health work applies the findings of social medicine to improving services and the health of populations. Social medicine is often said to be made up of social epidemiology, social psychology, medical geography, medical sociology and health economics. In some places, it is considered the academic arm of public health whilst others have called it the bridge between medicine and public health.
If you wish to find out more about social medicine, see The Society for Social Medicine.
There is also a journal Social Medicine which is full of interesting articles on the latest research in the field.
Linked to this is the Social Medicine Portal which can tell you more about the field, especially the work being done in the United States.
An interesting introductory read is http://www.med.unc.edu/socialmed/about/madison-1993 which explains how social medicine evolved from being an application of medical insights to social problems in 19th century to the multi-disciplinary field today.