"Overlay" (sometimes called "reveal") refers to the amount of cabinet face frame that is covered by the doors and drawer headers. International+ overlay doors and headers are larger, covering more of the frame. The result is a sleeker, tighter look. Traditional overlay doors and headers are smaller, revealing more of the frame. Since the smaller Traditional styles use less material, they cost less, too. And some styles have no overlay at all. "Inset" doors and headers fit flush with the cabinet's frame for a completely different look.
Learn all about inset cabinetry here.