An example of SDI is Notepad where you can only have one document open at any given time. If you want multiple documents and still want to use Notepad, you have to open up multiple instances of Notepad itself. This can be inefficient and for larger applications impractical. MDI uses a single primary window, called a parent window, to visually contain a set of related documents or child windows. Each child is essentially a primary window, but is constrained to appear only within the parent not the entire desktop.
Children typically share the menu bar, tool bar, and other parts of the parent's interface.
Secondary windows like dialog boxes are generally not constrained. Notice that MDI doesn't require your documents to be of the same type. Text, Image, Sound, etc, can all be simultaneously contained within the same parent. You are still free to present multiple views of the same data. To show you how easy MDI is with WFC, we built a Generic MDI application.