The RTOS stands for real time operating system. The RTOS is an operating system which is designed for real time applications that follow the real time criteria, efficiency, timeliness and expectedness. It is used in real time applications and support scheduling method which guarantees response time mainly to critical task. It is an operating system which supports real time applications and embedded system by providing correct result within the deadline required. The RTOS is also responsible for managing the hardware resources of an embedded system and hosting application which run on the embedded system. It gives service to guarantee deadlines will be met, scheduling algorithms in order to enable deterministic behavior in the system and perfect predictability than throughput. The important features of real time operating system are timeliness reached during proper scheduling algorithms, fault tolerance, design for peak load, predictability and maintainability. The implementations of some RTOS are very complex and very robust, while other implementations are very simple, and suited for only one particular purpose.