Your question is vague, but let's start here from IBM's description:
http://www.ibm.com/developerworks/rational/library/content/RationalEdge/sep04/bell
Search for aggregation and composition.
Basically, aggregation is used for a class with multiple dependent components, like a car and its parts. When a car is totaled, the wheels can be removed. The wheels and may have other purposes (inventoried, repaired, recycled, etc.). Composition exists when a component's lifecycle is dependent on the parent class lifecycle. An example of composition is a business and its operations (like its loading dock). When the business closes, the loading dock is no longer necessary so both instances are deleted.