The subject you need to look into is called a
Basically, a dynamically generated page (eg a CMS page) needs to get parameters sent to it via the URL so that it knows what content to display. For example, if a site only needs to know the article id in order to get that article's content from the database, then that page may initially have the URL site.com?id=24.
A rewrite engine could tidy that URL up so that it becomes site.com/24 or site.com/anything/24. It just needs to know the "24"
It could even remove the id, just so long as a database query could work it out from its replacement. For example... site.com/articletitle could allow a database query to ask "what is the id of the article with the title articletitle", before generating that page.
The example you give is pretty poor, as the date is back-to-front. However, the wikipedia entry has some nice examples of it in proper use:
(It is very unlikely that a static site would use such a directory structure)