The article Element

The article element is similar to the section element, but there are some notable
differences. Here’s the definition according to the spec: 

The article element represents a complete, or self-contained, composition in a document, page, application, or site and that is, in principle, independently distributable or reusable, e.g. in syndication.

The key terms in that definition are self-contained   composition and independently distributable. Whereas a section can contain any content that can be grouped thematically, an article must be a single piece of content that can stand on its own. This distinction can be hard to wrap your head around, so when in doubt, try the test of syndication: if a piece of content can be republished on another site without being modified, or if it can be pushed out as an update via RSS, or on social media sites such as Twitter or Facebook, it has the makings of an article. 

Ultimately, it’s up to you to decide what constitutes an article, but here are some suggestions in line with recommendations in the spec:

■ a forum post■ a magazine or newspaper article■ a blog entry■ a user-submitted comment on a blog entry or article

Finally, just like section elements, article elements can be nested inside other article elements. You can also nest a section inside an article, and vice versa. It all depends on the content you're marking up.