Quite honestly, and this is probably just me, but I consider a demon to be just that—a demon—but I consider devils to be demons that are generally malicious. It's kinda like the whole rectangle-to-square relation for me in the sense of a devil is a demon but a demon is not necessarily a devil, ya know?
However, I'm not sure if there is an actual difference between the two, or if the names are just generally interchangable. But like I said, that's how I classify the two. I'm not sure if it's correct, but it's how I do it! x3
~Melari