From W3C Wiki
The <aside> element represents a section of a page that consists of content that is tangentially related to the content around the aside element, and which could be considered separate from that content.
- Example of contents
- pull quotes
- groups of nav elements
etc. Other content that is considered separate from the main content of the page. [Example A]
See global attributes.
The following example shows how an aside is used to mark up background material on Switzerland in a much longer news story on Europe [try it]:
<aside> <h1>Switzerland</h1> <p>Switzerland, a land-locked country in the middle of geographic Europe, has not joined the geopolitical European Union, though it is a signatory to a number of European treaties.</p> </aside>
The HTML5 specification defines the <aside> element in 4.4.5 The aside element.