Joomla is a large framework designed to support small and large websites.
In the back-end of the site, information is stored in content items. There are two types of content items:
1. Content Items (also called Articles) organised into in Sections and Categories, and
2. Static Content Items (Only in Joomla 1.0)
Sections and Categories are mostly for organising large amounts of pages in the back end. There are some other uses.
Content items are only displayed in a Menu system if a Menu link has been created that links to the content item (ie after the page has been created).
(Note: It is also possible to set up a Menu link - Blog style - that links to an overview of every page in a category. In that instance, simply creating a content item in the correct category will put it into the link).
The menu system is very powerful, allowing you to link directly to a page on your site, to any other website address (URL), simply putting in a placeholder and many more features.
Your website can also have many plug-ins, such as shopping carts and form builders.