When you have chosen the settings you want in a dialog in the program, you can either confirm your choices by clicking on OK or Apply or simply Cancel the operation.
When you click on OK, you confirm your selection and the dialog is closed. Select OK if you are certain that you have chosen the right settings.
Clicking on Cancel will delete the settings you have made and then closes the dialog.
If you are not sure about your settings, first test them by clicking on Apply. The settings will now be altered, however the dialog will remain open. You can repeat this step until you are happy with the settings and then click on OK to confirm.