Using a CMS really shortens up the amount of time it takes to build content on your website. Joomla uses a system of Articles and Categories to manage your content. An article is like a paper you write for school and a category is like a divider in the binder you keep it in.
In Joomla 1.5 and earlier Sections would be like the whole binder to hold all of your content. That changed in Joomla 1.6. Now you can simply put categories inside of categories; adding limitless depth to a topic.
Use articles to publish your content to a page. This is an individual page with a title and specific information about. Articles are theoretically do not have other articles as a part of them. They are individual and usually occur under a category.
Categories hold all of the articles for a given topic. As of Joomla 1.6 categories can have sub-categories adding unlimited divisions of the topic. In some cases this makes things easier to manage and in other cases it makes things harder. It depends on the amount of content on the website.
Joomla sections are the top level for categories. All categories had to fit into a section. Now you can have a top level category or not. If there is no category, an article will be put in the “Uncategorized” category.
The Joomla documentation gives the example that if you had a category about “Dogs”, the section that it is kept in would be “Pets”.