Please could someone help?

I dont understand what I am doing wrong here. all I am wanting to do is have a div box with 2 divs in side it…one for content and the other is a side bar. Simple(I woulda thought but i am being proven wrong). I am a complete newb when it comes to CSS and for the life of me I cant figure out why the background div is not filling up the 100% as I am wanting.
Please if someone could have a look over my code and explain what I am doing wrong I would really appriciate it.
here is a link to the site:
http://www.arctosdesign.com/layout/index1.html

if you are using the firefox developers plug you should be able to veiw the css aswell.

I am use to tables and I am finding this quite confusing the whole when to use relative or absolute. and no matter what I try I just cant seem to get it. its how I want but with out the background Div behaving as it should.

thanx