In DataBase Modelling design each table should have a primary Key, a unique value to identify an individual row.
A Key formed by a unique value of a row that is made of attributes that already exist in the real world. such as SSN (Social Security Number), ISBN (International Standard Book Number) number or customer number (should be unique ).
A Key provide by the Data Base an integer, auto incremental. this is a preferred method for an entity type.
A key formed by to two values in columns to form one unique. the is the one value does not identify a row. one place you will see them often is when they are used in joining tables together to create to create many to many relationships.