What is the difference between candidate key and primary key

Primary key	Candidate key
(i) There can be only one primary key in any relation.	(i) There can be more than one candidate key in a relation.
(ii) No attribute of a primary key can contain NULL value.	(ii) The attribute of a candidate key can have NULL value.
(iii) It is optional to specify a primary key for any relation.	(iii) There cannot be a relation without candidate key specified.
(iv) Primary key describes the important attribute for the relation.	(iv) Candidate key present candidates that can qualify for primary key.
(v) A primary key is a candidate key.	(v) But it not compulsory that each candidate key can be a primary key.