Early computers could perform only one job or task at a time. This is often called single-user batch processing. The computer runs a single program at a time while processing data in groups or batches. In these early systems, users generally submitted their jobs to a computer center on decks of punched cards and often had to wait hours or even days before printouts were returned to their desks.
Software systems called operating systems were developed to make using computers more convenient. Early operating systems smoothed and speeded up the transition between jobs, and hence increased the amount of work, or throughput, computers could process.
As computers became more powerful, it became evident that single-user batch processing was inefficient, because so much time was spent waiting for slow input/output devices to complete their tasks. It was thought that many jobs or tasks could share the resources of the computer to achieve better utilization. This is achieved by multiprogramming. Multiprogramming involves the simultaneous operation of many jobs that are competing to share the computer's resources. With early multiprogramming operating systems, users still submitted jobs on decks of punched cards and waited hours or days for results.
In the 1960s, several groups in industry and the universities pioneered timesharing operating systems. Timesharing is a special case of multiprogramming in which users access the computer through terminals, typically devices with keyboards and screens. Dozens or even hundreds of users share the computer at once. The computer actually does not run them all simultaneously. Rather, it runs a small portion of one user's job, then moves on to service the next user, perhaps providing service to each user several times per second. Thus, the users' programs appear to be running simultaneously. An advantage of timesharing is that user requests receive almost immediate responses.