Motivation
Maps an association between objects to a foreign key reference between tables.
Summary
A Foreign Key Mapping maps an object reference to a foreign key in the database.
Parent object stores the list of child object collection mainly implemented ad lazy load.
Most of the ORM Framework like EF provides out of the box implementation as navigational properties and usually you should avoid custom implementation.
When to Use
A Foreign Key Mapping can be used for almost all associations between classes.