Some software applications are installed with an operating system installation and either depend on the operating system to function properly or help the operating system function properly. These kinds of applications are sometimes called system software because they are installed and, in a way, part of the operating system.
System software can be essential to the functionality of other applications as well and not just the operating system. Not all system software is used directly by users and works in the background. Others are used in the forefront, like Microsoft Internet Explorer (IE). While IE is not required for Internet browsing by Windows users, there are a number of other applications that rely on components of IE for the proper functionality, including parts of Windows.