<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.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
