Whats the difference between an chest x-ray and ct scan?

Both use x-rays: A chest ct is a very detailed look at the lung, blood vessels, heart and bony thorax. It takes very thin (3-5mm) slices which allows for detailed analysis of lung structure. The chest x-ray takes a 3-d structure and collapses it to 2-d. This still provides very detailed information of the chest, but can lead to overlap -- sometimes requiring additional views or a ct scan to clear things up.

One view versus many: A chest ct scan uses multiple low-energy x-rays from many directions to re-create a three-dimensional image of the body's density and display it in multiple "slices”. A chest radiograph uses low-energy x-rays from one or 2 directions producing a image much like a shadow. Unfortunately, this results in overlap of many structures. Additionally, structures with similar density can not be separated.