In simplest terms, an operating system is a collection of programs that manage a computer system's internal workings— its memory, processors, devices, and file system. Mainframe operating systems are sophisticated products with substantially different characteristics and purposes.

Operating systems are designed to make the best use of the computer's various resources, and ensure that the maximum amount of work is processed as efficiently as possible. Although an operating system cannot increase the speed of a computer, it can maximize use of resources, thereby making the computer seem faster by allowing it to do more work in a given period of time.

A computer's architecture consists of the functions the computer system provides. The architecture is distinct from the physical design, and, in fact, different machine designs might conform to the same computer architecture. In a sense, the architecture is the computer as seen by the user, such as a system programmer. For example, part of the architecture is the set of machine instructions that the computer can recognize and execute. In the mainframe environment, the system software and hardware comprise a highly advanced computer architecture, the result of decades of technological innovation.

Most of this information center teaches the fundamentals of z/OS®, which is IBM's foremost mainframe operating system. It is useful for mainframe students, however, to have a working knowledge of other mainframe operating systems. One reason is that a given mainframe computer might run multiple operating systems. For example, the use of z/OS, z/VM®, and Linux® on the same mainframe is common.

In addition to z/OS, four other operating systems dominate mainframe usage: z/VM, z/VSE™, Linux for System z®, and z/TPF.