Prospective and retrospective refer to when the study is done in relation to the study base experience (other epidemiologists refer to concurrent and non-concurrent).
Retrospective
A case-control study is carried out after the disease experience of the study base has already occurred or retrospectively.
Cohorts can also be retrospective. Looking back in time, a group of individuals is identified as a cohort (a typical example is a group of workers, such as shipyard workers in WWII) and then their disease experience over a period of time is investigated.