<header>
is one of several new tags in HTML5 that are supposed to replace <div>
for some specific situations. In particular, the “header” part of your page – whatever that is, usually the part that would be wrapped in <div class="header">
– in HTML5 you should use <header>
instead.
Chapter 3 of Dive into HTML5 by Mark Pilgrim does an excellent job going into the details of when and why to use the new <header>
element.