Case control study :
Case control:

A retrospective study.
Risk factor is to be identified — disease present 
Gives Odds ratio.
Best method to study RARE disease — Case control study.

NESTED Case-control study

It is a case-control study nested within COHORT study. So, it is a prospective study.
Is a prospective study
Gives Odd ratio
Useful in case of rare disease
Both 2 and 3