Relational Database Management Systems - RDBMS
A relational DBMS stores information in a set of "tables", each of which has a unique identifier or "primary key". The tables are then related to one another using "foreign keys". A foreign key is simply the primary key in a different table. Diagrammatically, a foreign key is depicted as a line with an arrow at one end.
In the example below, "Customer ID" is the primary key (PK) in one table and the foreign key (FK) in another. The arrow represents a one-to-many relationship between the two tables. The relationship indicates that one customer can have one or more orders. A given order, however, can be initiated by one and only one customer.