CMS, or Conversational Monitor System, is an operating system that runs primarily on IBM mainframe and mid-frame computers. It is a high-capacity environment that supports large numbers of interactive users. It features include: the ability to write, test, and debug applications, create and edit data files, process jobs in batch mode, share data with guest systems, and communicate with other system users.
CMS also supports a wide range of high-level languages and application environments. Some of its application development facilities include: extended architecture support, CMS Pipelines, multitasking services, Reusable Server Kernel, assembler macros and functions, OS/MVS simulation, and DOS/VSE support.
You often see the OS written as VM/CMS instead of just CMS, because CMS runs on top of another operating system, namely VM or Virtual Machine. The latest version of the IBM z/VM operating system provides CMS as a base component.