Super keys of a relation can have extra attributes. Candidate keys are minimal super key, i.e. such a key have no extraneous attribute. An attribute is known as extraneous if even after removing it from the key, create the remaining attributes still has the properties of a key.
The following properties must be satisfied by the candidate keys:
A relation can have more than single candidate keys and one of them can be chosen as a primary key.
For instance, in the relation PERSON the two likely candidate keys are PERSON- ID and NAME (assuming unique names in the table). PERSON-ID may be taken as the primary key.