A case-control study is a type of cross-sectional study used in medicine.
Typically, such studies are used to investigate aetiology of a given disease or outcome. Patients with the disease are identified and their exposure to a putative risk factor determined. The incidence of exposure to a the hypothesised risk factor is compared to that in a broadly similar, but healthy, group of subjects from the general population (i.e. a control population matched for other baseline variables that might confound the association). The results are expressed as an odds-ratio.
Cross-sectional studies are useful for investigating diseases of low prevalence, where recruiting the number of subjects required to show a significant difference between exposed and control groups in a longitudinal study (such as a cohort study) would be a protracted and impractical affair.
However, the latter methodology is considered superior in the hierarchy of evidence, because it is prospective and therefore not subject to forms of bias, such as recall bias, which afflict the former methodology.