The web today contains a ocean of news articles and blog entries. That gives W3C a good reason to define an element for article instead of <div class="article">.
The "article" element is meant for article of a web document. It is very common to have more than one articles in one web page.
What you can see below is a web page that contains more than one article in a page. However, when you click title, you will be brought to a page with one (1) article.
We should use article for content that we think it can be distributable. Just like news or blog entry can we can share in RSS feed.
"article" element can be nested in another "article" element. A typical example is user-submitted comments of a blog entry. the inner article elements represent articles that are in principle related to the contents of the outer article. For instance, a blog entry on a site that accepts user comments could represent the comments as another article elements nested within the article element for the blog entry.
In HTML5, It is perfectly okay to have h1 to h6 in an article element. As article element is a sectioning content, which means, it will be a subsection of its ancestor.
In our case, the article element is the subsection of the body element. Hence, it can have its own outline. With such, it is easier to maintain our web document, because you do not need to think about the outline outside of the article element.