Usable means 'able to be used'. That is usually something practical, like a part (say for a car). It can also be used for things like (say) some writing, which can be incorporated into something like a longer article, or perhaps a form.
Useful is a little more general. Its meaning is more 'could be used', or 'has benefit'. So an idea can be useful, someone can be useful, particularly if they do something useful. If you found something on a beach, that you might use on (say) an artwork, you could say 'that might be useful'.