In the case of single document interface applications, only one document can be opened at a time during the execution. Thus, single document interface applications have a narrow focus. The Microsoft paint is another example of this type of applications. In notepad you can edit only one document (text file) at a time. When you want to edit more than one document at a time, you have to run the multiple instances of notepad program that is separate multiple notepad windows executing simultaneously.
The Microsoft word is more sophisticated ands more complicated than notepad program. Word gives a more advanced graphical user interface and text formatting features that are not available in notepad.Besides these above mentioned features, word also allows the user to edit more than one document through the same instance of application program. These documents may be a text file, a bitmap file, a compressed file etc. each document in the MDI application is called a child window and application main window is known as parent window. A child window works under the control of parent window and different child windows may have different functionalities.