MFC Question:

Modal dialog is one which will not allow u to access any thing until this dialog is active.

Call:
Dialog::DoModal()

And reverse of this ur modeless dialog.
Dialog::ShowDialog();

For Example:
Modal Dialog:
When we access Menu items such as Save as, Open, attach file, in any application, we can not able to access any part of the application execpt the active dialog.

When we open add remove program for uninstalling any application, u will get a Uninstallation dialog which will be modeles. bcz still u were able to access add remove programs. (this is probably in Vista. And in XP its modal dialog which they have used)