Suppose, you have a class named UserInformation . And with that class you are having a object instance named UserContactInfo

Class UserInformation {

UserContactInfo userContactInfo ;

}

This leads to aggregation. The UserContactInfo object is aggreated within UserInformation class. Both the classes have individual identity.

Now , suppose we have a class named Rectangle . And within that class you have Class Point as a object.

Class Rectangle {

Point point ;

}

Now , rectangle is a composition of points. Point class doesn't have any individual important . It is point that makes a rectangle . This is known as composition.

Hope it makes clear.

Aggregation differs from ordinary composition in that it does not imply ownership. In composition, when the owning object is destroyed, so are the contained objects. In aggregation, this is not necessarily true.

For example, a university owns various departments (e.g., chemistry) , and each department has a number of professors. If the university closes, the departments will no longer exist, but the professors in those departments will. Therefore, a University can be seen as a composition of departments, whereas departments have an aggregation of professors.