Joomla explains how they organize content on this page: http://docs.joomla.org/Understanding_sections,_categories_and_articles, but I don't recommend it unless you're interested in technical details. Sections and categories tell Joomla where to put your articles. Here's an example:

On the Bob Davids Chapter site, there is a section called "Research", with these categories:

• Information Requests
• Submission Requests
• Presentations

If you go to the site, you can see a "Research" tab on the main menu at the top. If you hover over that with your mouse, you will see three options on the menu that represent the three categories. Click on any category and you will see all of the articles in that category.

That's how we use sections and categories; they let us split up the information and, more importantly, they let Joomla organize it for us so we don't have to. If you get a new Information Request and you want to post it, all you have to do is create a new article (see below) and put it in the "Research" section and the "Information Requests" category, and it will end up in the right place.