Help a newbie out!

I am still not sure what 5/8 means or any of those size terms.

Somewhere on the web, I saw drawings of cross sections of 5/8, 6/8, ect ect, all the sizes. At the time, I was overwhelmed with information and figured I would study that later. But now I can't find that link! Does anyone know where to look, I searched in here, as well as google.