Process scheduling is a technique which is used when there are limited resources and many processes.And only one process can be run at a time.So which process will be execute first is decided by process scheduling.If the system has more than one processor, then it is possible to execute more than one process at the same time.
Operating systems may have up to three distinct types of schedulers: long-term scheduler or high-level scheduler,medium-term scheduler and a short-term scheduler, The names suggest the relative frequency in which they are performed.