The difference between a case control study and a cohort study is, most importantly, one of direction. In a case control study, you start with individuals with known outcomes (cases and controls) and investigate to see what their different risk factors might have been. In a cohort study you start with individuals with known risk factors (exposed and non-exposed) and follow them to see their outcomes.
Case control studies are particularly useful for the investigation of rare diseases, since you can start with all the cases you can find and then choose appropriate controls. Prospective studies are not as useful for rare outcomes, because if the event of interest only occurs in one out of 1,000 or even 10,000 people, you don't have a very good chance of catching it in your selected study group
