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The aside element denotes content that is only tangentially related to the surrounding element. This is similar to a sidebar in a book or magazine.
The article element represents a self-contained piece of content in an HTML document that could, in principle, be distributed or used independently from the rest of the page.
The nav element denotes a section of the document that contains links to other pages or to other parts of the same page. Obviously, not all hyperlinks have to be in a nav element.
The header element denotes the header of a section. It can contain any content that you wish to denote as being the header, including a masthead or logo.
The section element is new to HTML5 and, as its name suggests, denotes a section of a document. Check out all properties about it.