The article element represents a complete, or self-contained, composition in a document, page, application, or site; that is, in principle, independently distributable or reusable, e.g., in syndication.
I gave my usual answer: think of not just as a newspaper article or a blog post, but as an article of clothing — a discrete entity that can be reused in another context. [...] It means an article represents content that can be taken out of the document and away from the immediate surrounding content, dropped somewhere else, say on another page, and still make total sense as it is grouped.
The section element represents a generic section of a document or application. A section, in this context, is a thematic grouping of content, typically with a heading.
A general rule is that the section element is appropriate only if the element’s contents are listed explicitly in the document’s outline. [...] a section is a part of a larger group without which it may not necessarily stand to make complete sense alone