Candidate key a attribute or set of attribute which is minimal and provide unique identification.
and set of all candidate key we choose appropriate key which is primary key . also note that all primary key is candidate key but all candidate key need not primary key.
in case of super key it also provide unique identification but it may be redundant or not minimal.
For ex-: suppose we have a relation with attribute A,B,C,D, in this AB can be a primary key and candidate key.
but ABC will be super key.