You use composition when you have a class that has a set of another objects, in any quantity. Use generalization when you have a class that shares common properties with a set of objects, but can also have other diferent properties or behavior.

For example, a Car has components like the engine, wheels, etc. This a composition relationship. But a Vehicle is a generalization of a Car, because you can have another types of Vehicles with diferent properties, like a Truck. Car and Truck are derived classes of Vehicle.