Nested case-control study can be described as follows: for a particular disease, all the patients that become diseased in a given cohort are labeled as "cases". Then corresponding to each "case", a pre-specified number (say, 4) of "controls" or healthy subjects (at the time when disease occurred for the case) are matched (irrespective of whether these healthy subjects became case at a later period). This design is interesting because cost can be minimized at the expense of negligible statistical inefficiencies compared to considering whole cohort. More can be found here.
