An Operating system (OS) is nothing but a collection of system calls or functions which provides an interface between hardware and application programs. It manages the hardware resources of a computer and hosting applications that run on the computer. An OS typically provides multitasking, synchronization, Interrupt and Event Handling, Input/ Output, Inter-task Communication, Timers and Clocks and Memory Management. Core of the OS is the Kernel which is typically a small, highly optimised set of libraries. Real-time systems are those systems in which the correctness of the system depends not only on the logical result of computation, but also on the time at which the results are produced. RTOS is therefore an operating system that supports real-time applications by providing logically correct result within the deadline required. Basic Structure is similar to regular OS but, in addition, it provides mechanisms to allow real time scheduling of tasks.