Excellent flowchart: http://html5doctor.com/downloads/h5d-sectioning-flowchart.png
Group together thematically-related content
Ask yourself - "Is all this content related?"
Eg: Layout split into 'intro' section, 'news' section, 'contact' section
Ask yourself - "Could this content belong as an entry within an RSS feed?"
Eg: Blog post, comment, etc
Represents a group of introductory or navigational aids and usually contains the section's heading
Most sections and articles have one. Can also belong in aside and header.
Contains information about its parent element, eg: who wrote it, copyright, links to related content, etc
NOT a description of the position. Can also belong in aside and header.
Holds related content
Ask yourself - "Can this be removed without reducing the meaning of the main content?"
Just because the content appears on the left or right of the main content does not make it an aside
Eg: within an article; related links, pull quotes, glossaries Eg: outside an article; sidebars, blog roll, advertising
Intended for major navigation information
A group of links grouped together isn’t enough reason to use the nav element
Only used once per page