Very simple.

semantically, all sets are made of subsets, right? Therefore:

-The aggregation is when those subsets exists independiently of the father set. As a monitor can be unplugged from the computer to be connected to another.

-The composition is when those subsets depends of the existance of the father set. As a leaf is a part of a tree or liver is a part of a body.

Those concepts talks about the kind of dependency between two objects or classes, conceptually. Directly in a program, in an aggregation, when the father object disposes, the aggregate objects should be disposed too. In the same scenario for a composition, comosite son objects will persist then the father object disponses.

Martijn Pieters post a pretty code example for that. Regards!!!

Semantically, all sets are made of subsets, right? Therefore:

• The aggregation is when those subsets exists independently of the father set. As a monitor can be unplugged from the computer to be connected to another.

• The composition is when those subsets depends of the existence of the father set. As a leaf is a part of a tree or liver is a part of a body.

These concepts talks about the kind of dependency between two objects or classes, conceptually. Directly in a program, in an aggregation, when the father object disposes, the aggregate objects should be disposed too. In the same scenario for a composition, composite son objects will persist then the father object dispenses.