An object of one class act as a Data member of another class. If a class has a reference of another class (also known as contained object, or entity reference), i.e. known as has-a relationship. For example Consider two Classes Employee and Address.
A relationship is a two-way association between two objects. Relationships associate objects with other objects. Create relationships to link objects with each other, so that when your users view records, they can also see related data.