A primary key is a special type of unique key that is used to identify a single row in a table. It is usually composed of one or more columns that contain only unique values, and cannot be NULL. For example, a table of employees may have an Employee ID column as its primary key.
A unique key is any combination of columns that contains only unique values. It is used to enforce data integrity and can be composed of one or more columns. For example, a table of customers may have a combination of first and last name columns as its unique key, ensuring that no two customers have the same name.