Fixed Header is the header that is always visible to users even the user scrolls down. It uses the fixed-position in CSS.
There a lot of websites nowadays that use a fixed header, such as gmail or Facebook. When you've logged into Facebook you scroll down you still see the header with the search bar and some other functions at the top. Similar functionality exists in gmail, but not in yahoo mail.
The benefit of a fixed header is that if the user need to use a function they can easily jump to the header without the need to scroll up, as would be required for a non-fixed header. However, if there is more content then users might lose some space because the header occupies some permanent screen real-estate. Also using fixed position CSS is not easy to do and so if you implement incorrectly the page can render incorrectly due to Fixed Position.
Web2.0 is website that interact with end user more, user can do all sort of things like move image around... like facebook opr tumblr.
So do users prefer a fixed header or non-fixed one in Web2.0?