Is there any difference between the primary key and unique key?

There is, of course, a difference between the primary and unique key

Primary key

• It enforces the uniqueness of column in a table.
• It does not allow the nulls.
• It chooses default cluster index

Unique key

• It allows one null value.
• It doesn't enforce uniqueness of a column in a table.
• It chooses a default non clustered index.