In any OS (Operating System), there are OK and Apply buttons. We saw them when trying to modify some internal settings of the OS itself or an application which is installed in that OS. The Difference OK Button Applies the changes and closes the dialog (or goes back to the previous location/one level up). Apply Button Applies the changes, but keeps the dialog open, so user can make further modifications. Therefore, if you wanna save the current changes you made and then close the dialog right away, hit the OK button. No need to hit Apply button then OK button. Just one time, hit the OK button. But, if you wanna save the current changes you made then wanna make some further changes, then hit the Apply button. How about Cancel and Close (or X ) buttons? It depends on the pop-up dialog you're at. They can have the exact same function, that is to close the window/dialog . Other case, such as in a document/text editing program or ot...