Why does Microsoft Windows has an APPLY and a OK button

I used to think about this question all the time, i know its kind of a dumb question but, there is no such thing. So here is your answer:  Sometimes when you are editing a setting example when editing permissions you want to apply a change test it "maybe" but keep the window open in case you want to make more changes, pressing OK will apply the changes, but it will  close the window.