Let's start with some definitions. In Joomla!, an Article is some written information that you want to display on your site. It normally contains some text and can contain pictures and other types of content. For many Joomla! sites, articles form the majority of the information presented in the website.
It is important to understand that, with Joomla!, the content of the site (for example, Articles) is totally separate from the formatting of the site — the way it looks on the page. So it is best to think of articles as pure content, independent of the way it might be presented. For example, the same Article might be shown with different fonts, colors, headings, and background, and might be shown in different locations on the page.