1) the #header needs a left-attribute, then you will see that its positioned to the browser window.
2) if you set position:absolute you need a width like wrapper has.
i made a jsfiddle, where you can see it and placed another div inside wrapper. just test a little bit and you will see how it works: http://jsfiddle.net/s9e9U/
and finally you can take a look at https://css-tricks.com/video-screencasts/110-quick-overview-of-css-position-values/ if you havent already