site/123, site/1/2/3, site/12/3..etc...

On one of my favorite websites, i've noticed something,

You can use any of the following URLS to gain the information, but what is the var name that it passes?

so I can then do with the var (1725785) what I wish.

It talks about ways to do this sort of thing with mod_rewrite and/or PHP. Unsure how the site does it above but maybe they just strip out /...