The section element divides the main content into sections. Each section should typically have its own theme and heading. A section can have its own header and footer but it is not required.
The section element represents a generic section of a document or application. A section, in this context, is a thematic grouping of content. The theme of each section should be identified, typically by including a heading (h1-h6 element) as a child of the section element.HTML 5.1 Nightly
Examples of sections would be chapters, the various tabbed pages in a tabbed dialog box, or the numbered sections of a thesis. A Web site's home page could be split into sections for an introduction, news items, and contact information.
Authors are encouraged to use the article element instead of the section element when it would make sense to syndicate the contents of the element.
If an article contains content that is only one part of the page it could also be a child of a section