"Useful" is a common word, "usable" is rare. A screwdriver is a very useful tool. Every homeowner should own a set of screwdrivers. A screwdriver that is the wrong size may be usable, but it is much better to use one that fits correctly.

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'.

Thank you!

1.Useful things have positive effects, they help you to do something easier. 2.This is about -able ending: http://www.oxforddictionaries.com/words/words-ending-in-able-or-ible