Source : Free On-Line Dictionary of Computing

multitasking
     
         (Or "multi-tasking", "multi-processing",
        "multiprogramming", "concurrency", "process scheduling") A
        technique used in an {operating system} for sharing a single
        processor between several independent jobs.  The first
        multitasking operating systems were designed in the early
        1960s.
     
        Under "{cooperative multitasking}" the running task decides
        when to give up the CPU and under "{pre-emptive multitasking}"
        (probably more common) a system process called the
        "{scheduler}" suspends the currently running task after it has
        run for a fixed period known as a "{time-slice}".  In both
        cases the scheduler is responsible for selecting the next task
        to run and (re)starting it.
     
        The running task may relinquish control voluntarily even in a
        pre-emptive system if it is waiting for some external {event}.
        In either system a task may be suspended prematurely if a
        hardware {interrupt} occurs, especially if a higher priority
        task was waiting for this event and has therefore become
        runnable.
     
        The scheduling {algorithm} used by the scheduler determines
        which task will run next.  Some common examples are
        {round-robin} scheduling, {priority scheduling}, {shortest job
        first} and {guaranteed scheduling}.
     
        Multitasking introduces {overheads} because the processor
        spends some time in choosing the next job to run and in saving
        and restoring tasks' state, but it reduces the worst-case time
        from job submission to completion compared with a simple
        {batch} system where each job must finish before the next one
        starts.  Multitasking also means that while one task is
        waiting for some external event, the {CPU} to do useful work
        on other tasks.
     
        A multitasking operating system should provide some degree of
        protection of one task from another to prevent tasks from
        interacting in unexpected ways such as accidentally modifying
        the contents of each other's memory areas.
     
        The jobs in a multitasking system may belong to one or many
        users.  This is distinct from {parallel processing} where one
        user runs several tasks on several processors.  {Time-sharing}
        is almost synonymous but implies that there is more than one
        user.
     
        {Multithreading} is a kind of multitasking with low
        {overheads} and no protection of tasks from each other, all
        threads share the same memory.
     
        (1998-04-24)