In HTML 4, we use the <div> tag to separate HTML pages into parts. Sometimes those parts were structurally meaningful. For example, a page describing a course might include an overview, goals, prerequisites, and an outline. Each of those parts might be enclosed in a <div> tag with meaningful ids to provide meaningful structure to the page. However, <div> tags are also used to separate parts of a page for styling purposes, for example to create a column layout. In this case the areas encompassed in <div> tags might not be structurally different. Browsers cannot distinguish between structurally meaningful and meaningless divs, so they do not attribute any special significance to either kind.
This lesson explains the purpose of and difference between <section> and <article> tags, how they differ from <div> tags and how they affect a page's "outline."