Foreign key

The foreign key identifies a column or a set of columns in one (referencing) table that refers to a set of columns in another (referenced) table. The columns in the referencing table must be the primary key or other candidate key in the referenced table.

Difference between primary key and unique key

Primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist and no null values are entered.
Unique key constraint is used to prevent the duplication of key values within the rows of a table and allow null values. (In oracle, one null is not equal to another null).