Association: The object which have a relation with eachother are equal. For example a person drives a car.
Aggregation: One object is owner of the other object. For example a department and a person which is 'chief of the deparment'
Composition: One object is owner of the other object. But when the owner is deleted to other object is useless. For example a Room belongs to a house. When the house is destroyed the room doesnot exists anymore.
See for more details Wikipedia
