╥Cancel╙ means ╥dismiss this operation, with no side effects.╙ It does not mean ╥done with the dialog box,╙ ╥stop what you are doing no matter what,╙ or anything else.
In alert or dialog boxes, use Cancel for the name of a button that closes the alert or dialog box and returns the system to the state it was in before the alert or dialog box was displayed. When a lengthy operation is in progress, use Cancel for the name of a button that dismisses the operation and returns the machine to the state it was in before the operation began, with no side effects.
What to do the rest of the time
When it is impossible to return to the state that existed before an operation began, do not use the word Cancel. Two common alternatives, useful in different situations, are OK and Stop.
In alert or dialog boxes, use OK for the name of a button that closes the alert or dialog box and accepts any changes made while it was displayed. For confirmation alerts (alerts that say, essentially, ╥Are you sure you want to do this?╙) and many simple dialog boxes, it is better to use a word or two that succinctly describes what accepting the alert or dialog box means, such as Revert or Change All.
When a lengthy operation is in progress, use Stop for the name of a button that halts the operation before its normal completion, accepting the possible side effects. Stop may leave the results of partially-completed tasks around, but Cancel never does.
Some correct examples
Following is a series of examples of proper uses of Cancel and its cousins.
Figure 1╨Confirmation alert
The alert in Figure 1 uses Revert instead of OK, since Revert neatly sums up what accepting the alert means.
Figure 2╨More complex dialog with Cancel and OK
The dialog box in Figure 2 uses OK because there is no succinct term to describe what accepting the changes in the dialog box means.
Figure 3╨More complex dialog with OK instead of Cancel
The dialog box in Figure 3 uses OK because it doesn╒t throw away all the changes that were made since the dialog box was first drawn. If the button were named Cancel instead, clicking it would remove any formats created since the dialog box was drawn, bring back any formats removed since the dialog box was drawn, and undo any changes that had been made by selecting a format and clicking Modify since the dialog box was drawn.
Figure 4╨Progress indicator that uses Cancel
The dialog box in Figure 4 uses Cancel because clicking the button leaves the document named Wombat Data in the state it was in before the Insert File command was chosen.
Figure 5╨Progress indicator that uses Stop
The dialog box in Figure 5 uses Stop because clicking the button stops inserting text into the document named Wombat Data, but it doesn╒t remove the text that has already been inserted.
