In this category are all pretty simple. Catalog is the electronicreference library for resources (web sites) a certain direction. It's really all in this library - all sites placed in categories with brief descriptions for easy perception. The main disadvantage of this system is the so-called "manual set" of sites, ie, not all the pages of a global network available there, because for adding your owncreations to the directory that should be registered, and that takessome time.