Joomla! is a content management system. Sections and categories allow you to organize your content.

The basic structure is: Sections include Categories. Categories include content items (1.0)/articles (1.5).

Section A

Category A1
                content items/articles
Category A2
                content items/articles
Category A3
                content items/articles

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/articles 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.

Although it makes sense to organize your work into categories and sections, you do not need to show these to your users. Using your menus, you can link directly to sections, categories and content items. You can also select numerous options for the display of content associated with each type of link.