Categories and sub categories can be discussed as "virtual folders" and publications as files allocated inside this folders. So, to establish publications you are first to establish categories and/or sub categories for them. 

Easily add up to 3 levels of nested categories and sub categories

Easily Edit Categories and sub categories - rename, move, re-order.

After deleting categories publications inside thise categories are not automatically deleted, but they are marked as "Unassigned" publications and can be moved to other categories or deleted - while using "List Edit" option of Administration.

How to use categories as links to external web pages.