The words that appear on the alert boxes are sometimes ambiguous. For example, if the task is to quit the application, then an alert box with the pair of choices "Quit" and "OK", or with "Cancel" and "Continue" are ambiguous, and can be taken either way. Are there better words that can be used unambiguously in wide variety of situations?
"Do you really want to get answer to the question?"
so action buttons are either:
No/Not now/Cancel/Quit/Later sometime/Continue what I was doing
My point is that it depends on context also. So you need to choose right pair of positive and negative action label considering mood of the site/application. But the 'statement' for alert/message itself should be clear so that user will understand which will be positive and negative actions and will see the same below. They should compliment the purpose of the statement.
Thus 'Do you really want to do it?' will be very loosely framed statement which may be the starting point of confusion. If action button labels are not properly used, it adds to the confusion.
So designer should not rely ONLY on action button labels but statement/message should also be clearly conveyed.
I think you should have words suitable for every alert box. General ones usually do not work, but they are the standard: OK, and Cancel. Okay means do what you say you are going to do, cancel means take me back and do nothing...
For this example of closing an application, Quit and OK are not a pair, they both mean the same thing, and Continue and Cancel also both mean the same thing. So the right way to do is "Quit" and "Cancel"
You can be better by being more expressive, "Yes, quit application." "No, not yet."
Or "Quit application" "Take me back"
I suggest you stay away from "Continue" because continue really might mean "continue in quitting application"
My advice is never have an alert box that just asks someone whether they want to quit the application. This is perhaps the most annoying design decision in the history of computers, ever. Yes I want to close the app. Otherwise I wouldn't have clicked on the close button!
Asking "Are you sure you want to quit"? Is insulting to the user. Yet, it is amazing how many major software applications do this. Skype is a particular offender, requiring me to sign out, then click quit, then say that I am sure I really want to quit.
If there is unsaved data, you may need to have a dialog box that specifically asks about the unsaved data. "Do you want to save the file before closing?" with "Yes/No/Cancel" is one standard format that everyone should understand. Even then, though (depending on your specific use case) it may be better to auto-save the state so that you don't have to nag about it.
Please, please don't ask me if I am sure I want to exit!