Joomla CMS Question:

The basic structure is:
Sections include Categories.
Categories include content items.

Section A
Category A1
content items
Category A2
content items
Category A3
content items

Section B
Category B1
content items
Category B2
content items
Category B3
content items

Section C

Category C1
content items
Category C2
content items
Category C3
content items

The content items are what you would think of as web pages in a traditional html site.

Both section and category "pages" can be created to serve (more or less) as the home pages for the categories and sections, respectively.