CMS (Conversational Monitor System) is a product that comes with IBM's VM/ESA operating system and allows each of many simultaneous interactive users to appear to have an entire mainframe computer at their personal disposal. Almost as old as IBM's MVS operating sysem, VM/ESA (generally known as "VM") and CMS originated at IBM's Cambridge Scientific Center in 1964. VM provides an extra layer of programming below an operating system, called the control program that handles the actual machine operation of the computer. The control program lets each operating system, such as MVS and CMS, appear to be in sole charge of the computer - effectively, creating a virtual machine . CMS goes a step further and lets each CMS user appear to have their own personal operating system.
Users of a VM system typically log on to VM first and then (virtually) start their own personal CMS (by entering the command: i cms). Thereafter, the user can communicate with other active users, send mail, develop and run applications, and use many standard applications and tools that have been developed over VM and CMS's evolution.