Apple licensed parts of their designs to Microsoft for use in Windows 1 which was little more than a graphical front end for the DOS. With little interest in Windows 1 Microsoft worked to improve Windows 2 and make it more Mac like. Their efforts did not go unnoticed by Apple who sued them for copyright infringement. This failed as the judge ruled that Apple had licensed most of the disputed elements to Microsoft.
Despite such early similarities there are fundamental differences between the two systems. The Mac OS is a true windowing system. A window allows a user to observe and interact with the running application. If no further interaction is required a window can be closed but the application will continue to run. Windows runs the application in the window with the application's menus attached to the window. If the window is closed then the application quits.