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perhaps these are just different names for the same thing and it varies from operating system to operating system.
A scheduler control the execution of programs in a multiprogramming environment, i.e. in a single processor system decides which program of those able to run can execute using the CPU. For example
1. priority based - in a real time system tasks will be assigned priorities based on how critial they are. The scheduler runs the highest priority process that can run
2. in a time share system programs may run in a round robin system where each program gets a time slice to execute - so long as they are not too mant programs they all appear to run at the same time.
Some operating systems run multi-level round robin system with high priority tasks (e.g. IO bound) in the top level and low priority (e.g. CPU bound) in the bottom. If nothing can run in the top level round robin it moves down the priorities until it finds one it can run. If nothing can run the CPU idle task runs.