generic flow container
*div*—the "generic flow container" we all know and love. It’s a block-level element with no additional semantic meaning (W3C:Markup, WhatWG)
generic document or application section
*section*—a "generic document or application section". A *section* normally has a heading (title) and maybe a footer too. It’s a chunk of related content, like a subsection of a long article, a major part of the page (eg the news section on the homepage), or a page in a webapp’s tabbed interface. (W3C:Markup, WhatWG)
an independent part of a document or site
*article*—an "an independent part of a document or site". This means it should be able to ‘stand alone’, and still make sense if you encountered it somewhere else (eg in an RSS feed). Examples include a weblog article (duh), a forum post or a comment. Like *section* these generally have a header, and maybe a footer (W3C:HTML, WhatWG)