In the early years (1950-1965) IBM made many models of mainframes, many of which were incompatible with each other. It had two main lines of models: one for commercial or data processing use, and another one for engineering and scientific use. The two lines were largely incompatible. This all changed with the introduction of the System/360 in April of 1964. The System/360 was a single series of models, compatible with one another, for both commercial and scientific use. The System/360 later evolved into the System/370, the System/390 and most recently the zSeries.
System/360 (the "all-around computer system") incorporated into a single architecture features which had previously been present on only the commercial (decimal arithmetic, for example, or byte addressing) or the technical (floating point arithmetic) lines of machines. Some of these features were optional on some models of System/360.
System/360 was the first computer architecture in wide use to include provisions for the use of an operating system. Among these were the notion of supervisor and problem state programs and instructions, as well as memory protection.
The primary operating systems in use today on IBM mainframes include MVS (also called OS/390 or z/OS), VM/CMS (also called z/VM), VSE, TPF, and most recently Linux. (A few systems also remain that run MUSIC/SP). Historical operating systems for the System/360 and its successors have included OS/360, MFT, MVT and SVS.
Many of the pre-System/360 mainframes also had operating systems, but they were very rudimentary by today's standards.