Definition
A case-control study is a retrospective study that compares two groups of people: those with a specific outcome (cases) and similar people without the outcome (control)
Examples
Suppose that researchers want to study the relationship between smoking and lung cancer. They identify 100 lung cancer patients (cases) and 100 matched individuals without lung cancer (controls). They then collect data on past smoking habits for both groups and compare the prevalence of smoking between cases and controls.