tion:Distributed operation system (DOS): An operating system that runs over multiple devices usually connected by a network medium.Real-time operating system (RTOS): An operating system that can respond to and schedule events in a precious time; i.e. time response is highly predictable.The two categories are in FACT independent from each other; that is DOS is not directly related to RTOS. Though, one can argue that a distributed system is inherently non-real time due to the unpredictable nature of the communication medium.RTOS deals with how the OS will respond to events and external devices that requires handling in a SPECIFIC period of time or otherwise a deadline might be missed and the system can fail.For example, think of a car with cameras and SENSOR to detect pedestrians. If these sensors need to communicate with the CPU/OS for an urgent request regarding incoming pedestrians so as to decelerate the driving speed. RTOS in this case will respond in real-time exactly in the same time the request is ISSUED and hand over the CPU for this effort.Non real-time OS like Windows is not predictable. Each process takes a time slice to execute but that exact time slice is not known upfront.On a separate topic, a distributed system is software that can run correctly on multiple computing devices. So imagine multiple computers all running the same OS.The benefit is the ability to scale for example using more RAM from more devices and be able to continue working if a machine failed.Thanks Mark as BRAILIST