Описание тега nav

The <nav> tag is new HTML5 tag which defines a section of navigation links.

<nav>
   <ul>
      <li><a href="/html/">HTML</a></li>
      <li><a href="/css/">CSS</a></li>
      <li><a href="/js/">JavaScript</a></li>
      <li><a href="/jquery/">jQuery</a></li>
   </ul>
</nav>

In the HTML5 specification its primary use case is defined as follows:

The element is primarily intended for sections that consist of major navigation blocks. In particular, it is common for footers to have a short list of links to various pages of a site, such as the terms of service, the home page, and a copyright page. The footer element alone is sufficient for such cases; while a nav element can be used in such cases, it is usually unnecessary.

For more information on usage see HTML5 Doctor or the w3.org HTML reference.