difference between primary key and unique key

Primary Key:

• Can be only one in a table
• It never allows null values
• Primary Key is unique key identifier and can not be null and must be unique.

Unique Key:

• Can be more than one unique key in one table.
• Unique key can have null values(only single null is allowed).
• It can be a candidate key
• Unique key can be null and may not be unique.