The OnClosing is fired when the form is in the process of closing (after clicking on the Close button for example) but before the form is actually closed. This gives applications a chance to for example ask if the user wants to save the current document and cancel the closing of the form if the program requires it. OnClosed fires when the form is actually closed, which can be useful to do some cleanup.

You might want to read up on the obsolescence notice regarding these events though.