One of the most confusing elements of selling SharePoint is explaining the different structural elements (sites and libraries/lists) and their limitations (namely the infamous 10,000 and 5,000 item limits respectively).
If you’re in the business of selling or training SharePoint then you probably have your your own ways of explaining the above, but this one seems to work well for me.
For me the best way to think of SharePoint in storage terms is as a book store.
The store in this case is the intranet or overall team site; containing bookshelves (sub-sites), books (libraries), and pages (folders & files).
In this instance the store can contain a large number of bookshelves (10,000) and each bookshelf in turn can only contain a finite number of books (10,000) with each book only able to contain a finite number of pages (5,000).
If we have a book that contains more than 5,000 pages we simply break it into multiple books/volumes with meaningful and related names– e.g. Shared Data A-L and Shared Data M-Z.
Likewise, if we end up in a situation where there are more than 10,000 books (libraries) in a bookshelf (sub-site) on a specific subject we split the books across multiple bookshelves– but if this proves unwieldy or confusing for customers we simply set up a new store (site) specialising in those books (files & folders) and once again have the ability to have 10,000 bookshelves (libraries) around that subject.
Hope that all made sense…