What is the difference between PRIMARY KEY and UNIQUE?

Both PRIMARY KEY and UNIQUE enforce uniqueness, but they serve different relational purposes.

A PRIMARY KEY identifies the principal key chosen to identify each row. A table can have only one primary-key constraint, and its key columns cannot be NULL. A table can have multiple UNIQUE constraints, which are useful for alternate candidate keys.

In SQL Server, a primary key creates a unique index by default; whether that index is clustered or nonclustered depends on the table's existing indexes and the explicit constraint definition.