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 (we) can make further modifications. Therefore, if we want to simply save the current changes we've made ➡️ hit the OK button. 👋 No need to hit Apply button ➡️ OK button. 👍 Only once, hit the OK button ✅ But, if we want to keep the dialog open, then hit the Apply button. It will perform a similar function to OK button without closing the dialog . How about Cancel and Close (or X ) buttons? It depends on the pop-up dialog we're at. They can have the exact similar purpose, that is to close the dialog . Other case, such as in a document/text editing pro...