As far as the actual material, there is little difference between a fly line and a backing, with the exception of the overall diameter. The backing line is usually a smaller diameter, while the fly line is a bit larger. The real difference is how and where they are used. The fly line is the line that you cast out to the water with the lure attached. The backing is attached to the other end of the fly line and is used when you need more line to handle a larger fish that has grabbed the line.