From HTML WG Wiki
Sections of an HTML Document
A section is a part of something, a segment. Sectioning can be applied to various settings. A section is limited to a single subject or theme.
In writing, a book is sectioned into chapters. The chapters, in turn, are sectioned into paragraphs. Within a discourse, a quote from another author may be included or another complete article imbedded.
Within the body of a web page, the page title is a section, as are navigation bars, as well as the main content area.
The word section is used generically to refer to any of the six (6) HTML elements that are used to create sections. One of these is named the section element. The context of the word section should make clear if it is referring to the element or being used generically.
Note that the div element is not a section element. (But note that error correction may sometimes cause user agents to treat it as one.)
The section elements are
.Guide/body:body .section .nav .article .Guide/blockquote:blockquote .aside
The section elements are described individually elsewhere. The implications of various combinations and implementations are examined here.
[add combinations and implementations text]
h1 through h6 create an implicit section; the number should match the embedding level. If content is included (so the embedding level isn't known), user agents are most likely to correct the level of an h1 element.
re-reading the spec, h1-h6, header, footer, and address elements are not sectioning elements, although they should be used within sectioning elements. They have been removed from the list above.