It is often essential to organize applications into independent, though cooperating, programs. Each of these programs, while executing, is called a task.
With a preemptive priority-based scheduler, each task has a priority and the kernel ensures that the CPU is allocated to the highest priority task that is ready to run. This scheduling method is preemptive in that if a task that has higher priority than the current task becomes ready to run, the kernel immediately saves the current task's context and switches to the context of the higher priority task.
Preemptive priority scheduling can be augmented with round-robin scheduling. A round-robin scheduling algorithm attempts to share the CPU fairly among all ready tasks of the same priority. Without round-robin scheduling, when multiple tasks of equal priority must share the processor, a single task can usurp the processor by never blocking, thus never giving other equal-priority tasks a chance to run.
In normal OS, the result of the processing may or may not come and there is no guarentee that the result will be produced. It may not process the request in real time nor respond to any request in specified time. But in RTOS, there is a guarentee that the result will be there, and that too in specified time.