What is the primary key?

Candidate key.
It is a column that is use to uniquely qualify and identify a database record without referring to another data. One of the candidate keys is selected as the table primary key. Examples of primary key are: driver license, vehicle identification number and social security number.

A composite key is a combination of 2 or more columns in a table that can be used to uniquely identify each row in a table.

Determinant is any attribute that can be used to determine the values assigned to other attribute in the same row.
Foreign key is a column or columns that refer to primary key in another table.

A candidate key is a column or a row of columns in a table which identifies any database record without utilizing any other data. Each table can have one or more candidate keys, but one candidate key is unique, and it is called the primary key. The primary key is usually the best among the candidate keys to use for identification.

The candidate key must be one of a kind. For example, to identify each employee a company might use the employee’s social security number. Bank account numbers are also examples of candidate keys, as most people do not recycle bank account numbers.