When to include an Apply button?

The Apply button is most commonly used in dialogues that feature more than one tab/perspective. The apply button is there to ensure the user that changes being made in one tab will not be lost when the user switches to another tab.

1. What is the use of apply button?
2. What is the difference between OK and Apply button?

What is the use of apply button?

This is the function of the Apply button. The Apply button applies the current settings in all of the property pages to the external object, as opposed to applying only the current settings of the currently active page.

What is the difference between OK and Apply button?

The Apply button applies the changes to the settings that have been made, but does not close the dialog box. The OK button also applies the changes to the settings, but in addition closes the dialog box. Apply is useful if you are trying out different settings and don't want to have to keep re-opening the dialog box.