Case-control studies start with the identification of a group of cases (individuals with a particular health outcome) in a given population and a group of controls (individuals without the health outcome) to be included in the study. In a case-control study the prevalence of exposure to a potential risk factor(s) is compared between cases and controls. If the prevalence of exposure is more common among cases than controls, it may be a risk factor for the outcome under investigation. A major characteristic of case-control studies is that data on potential risk factors are collected retrospectively and as a result may give rise to bias. This is a particular problem associated with case-control studies and therefore needs to be carefully considered during the design and conduct of the study.

For more information see: 

Principles of Epidemiology in Public Health Practice, 3rd Ed. US Department of Health and Human Services, Centers for Disease Control and Prevention.

Chapter 8. Case-control and cross sectional studies from BMJ Epidemiology for the uninitiated

CASP: Case control study checklist