What is the difference between a composite class and a list or collection of classes?

I'm doing a project in Bluej and I have to develop a subsystem that tracks the populations of political units like nations, states, and cities and I need to use an interface in conjunction with a class called "city" with a given population and an ArrayList called "state" that takes the combined populations of all its cities and compare them. Attached is the assignment itself. Any help whatsoever would be awesome....