A z/OS subsystem of the IBM OS/390 and z/OS mainframe operating systems that manages jobs (units of work) that execute on the system. Each job is described to the operating system by system administrators or other users in job control language (JCL). There are two versions, JES2 and JES3. JES3 allows central control of the processing of jobs using a common work queue. OS/390 and z/OS provide an interactive menu for initiating and managing jobs.