MVS (Multiple Virtual Storage) is the most commonly used operating system on the IBM System 370 and System 390 mainframe computers. It was later renamed by IBM to OS/390, and then when 64-bit support was added to z/OS; but it remains fundamentally the same operating system. It is descended from SVS, which is in turn descended from MVT, MFT and finally OS/360.
OS/360 originally did not support multitasking. MFT (Multitasking with a Fixed number of Tasks) added multitasking, but only allowed a fixed number of tasks to execute at once. MVT (Multitasking with a Variable number of Tasks) was an enhancement to MFT that supported a variable number of tasks. SVS (Single Virtual Storage) added virtual storage (more commonly known outside IBM as virtual memory), with the same address space being shared by all tasks. Finally, MVS allowed different tasks to have different address spaces.
MVS originally supported 24-bit addressing; as the underlying hardware was extended it was also to progressively support 31-bit and now (as z/OS) 64-bit addressing.
The main interfaces to MVS are JCL (Job Control Language), the batch processing interface, and TSO (Time Sharing Option, since originally it was an optional part of the system), the interactive time-sharing interface. There is also a special interface used by operators at the system console. CICS is an interface widely used in the banking sector; while ISPF adds forms and menus to TSO.
MVS systems are traditionally accessed by 3270 terminals, or by PCs running 3270 emulators; many mainframe applications these days have custom WWW or Windows interfaces, but 3270 access is still necessary for programming and system administration.