The application used here is del.icio.us and not diigo but the way it's used is exactly the same. So it should help if you're unclear on this concept still.
Overview of the history, philosophy, and basic operations in set theory. Fortunately, sets are an extremely flexible way to describe mathematical objects, such as the sample space of some experiment. To our disappointment, it turns out to not be the be-all, end-all of mathematics, but that's a subject for a logic course.
Unless you're really interested in all the pictures of license plates, scroll down to "County coding" to see the mathematically relevant part. Ask yourself: how many license plates can be generating in the scheme used from 1963 to 2008? How many can be generated in the new scheme?
