The HTML5 standard is still under development and they recently made a change to the aside tag. Previously the content of the aside tag had to be used for content related to the article content. In the new specifications aside’s inside a article has to have content related to the article. If you use the aside tag outside the article the aside could be used as a regular sidebar. That is more or less what we have seen the aside tag has been used for so far anyway. That is the way I used it on my HTML5 projects so far.
... <body> <article> <h1>Blogpost</h1> <p>Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas.</p> <aside> <!-- Content related to the blog post. --> <h1>Specs</h1> <ul> <li>Lorem ipsum dolor.</li> <li>Aliquam tincidunt mauris eu risus.</li> <li>Vestibulum auctor dapibus neque.</li> </ul> </aside> </article> <aside> <!-- Content related to the site. --> <nav> <ul> <li><a href="#">Home</a></li> <li><a href="#">About</a></li> <li><a href="#">Clients</a></li> <li><a href="#">Contact Us</a></li> </ul> </nav> </aside> </body> ...