What is the difference between Primary key and unique key?

Primary key: A column in a table whose values uniquely identify the rows in the table. A primary key value cannot be NULL.

Unique Key: Unique Keys are used to uniquely identify each row in the table. These can be one and only one row for each unique key value. So NULL can be a unique key. There can be only one primary key for a table but can be more than one unique for a table.