A primary key is a field in a table that uniquely identifies each record in the table. It is a combination of a unique index and a not null constraint. For example, a customer table may have a primary key of customer_id.
A unique key is a field in a table that uniquely identifies each record in the table. It does not have to be the primary key, but it must contain unique values. For example, a customer table may have a unique key of email address.