In principle, the low-level software that handles the interface to peripheral hardware, schedules tasks, allocates storage, and presents a default interface to a user. The term OS has burgeoned to include applications, utilities, and “bells and whistles” sold or supplied with the OS and usually integrated into the OS package. As a result, the OS may be split into a kernel which is always present and various system programs which use facilities provided by the kernel to perform higher-level house-keeping tasks, and graphical user interfaces, low level applications, etc.
Thus, for example, Windows was originally a graphical interface application that ran on top of the MS-DOS operating system, but now is referred to, colloquially at least, as an operating system in its own right. Much of the Microsoft antitrust litigation concerned this integration as well as the integration of applications such as Explorer into the operating system.