The web server is configured to use something like mod_rewrite to pass everything after index.php as a query string parameter to index.php. Many sites use this (though most don't do it so nakedly) to make pretty URLs that search engines and users prefer.

Essentially, the request is equivalent to index.php?Article=2013%2F12%2Fhow-blender-started or perhaps index.php?Year=2013&Month=12&Article=how-blender-started.