Ofcourse Sheldon Brown has a good article on the subject: http://www.sheldonbrown.com/frame-materials.html
In general Al is softer and bends easier than steel but is much lighter. In pratice the lower density of Al allow larger tubes at a lower weight with a resonable wall thickness (preventing beer can syndrome) which gives a stiffer frame. However the trade offs between the different frame metals allow a good bike designer to make a stiff steel frame or a compliant Al frame or anything in between without a large difference in weight. CF allows are little more choice in properties due to the oriented strands but at the expense of manufacturing complexity.
Craig