Page content in Joomla 1.5 is organized by Sections, Categories & Articles like this:

• SECTION 1
  • CATEGORY 1
    • Article 1
    • Article 2
    • Article 3
  • CATEGORY 2
    • Article 1
    • Article 2
• SECTION 2
  • CATEGORY 1
    • Article 1
    • Article 2
    • Article 3
  • CATEGORY 2
    • Article 1
    • Article 2

Sectionsare the top level of organization - they are usually the main divisions of your site.

Categories are sort of sub-sections.

Articles are where you actually write your content - they can include text and images as well as any other media item you want on your site. You can move content items from one category to another so don't worry if you don't put it in the right place when you create it. Articles can be displayed as pages on your site. Some content is generated by components (such as VirtueMart) so if the content you are trying to edit is not listed under Articles, try looking in components.

Page content in Joomla 1.6 is organized by Categories & Articles, but no Sections.

Page content in Joomla 1.0 is very similar to v1.5 except Articles are called "Content Items".