OK, lets try to reason out the meaning and use of the terminology, since you're really interested. Besides empirical formula, what other types are there. How do they differ from one another for the same substance (pick a substance that best highlights the differences.) And then we'll have solved the problem.
[EDIT]
I'm just going to assume, that
billnotgatez:'s answer will still leave you with more, "but why?" questions. So its still worth working it out to your satisfaction.
[EDIT again]
Hey, where'd that answer go?