Real-time Operating System

This operating system used to get quick response. It is very fast and quick response system. These systems are used in an environment where large number of events must be accepted and processed in a very short time. Real time processing requires immediate response from the system.

In this system there is a little swapping of programs between primary and secondary memory. Most of time process is kept in primary memory.
Example: Air traffic control, defense application, system like RADAR, robotics etc.  

Real time system comes in two states:

HARD Real time systems

This system guarantees that critical tasks be completed on time. This goal requires that all delays in the system be bounded, from retrieval of stored data to the time that is takes the OS to finish any request made by it.
Such time constraints dictate the facilities that are available in real time systems. Secondary storage of any sort is usually limited or missing, with data instead of being stored in short-term memory or in ROM (Read only memory)

SOFT Real time systems

This system is used , where critical real time task get priority over the other tasks, and retains that priorities until its complete. As in HARD Real time systems, the OS kernel delays need to be bounded (A real time task cannot be kept waiting indefinitely for the kernel to run it).
SOFT Real time systems is an achievable goal that can be mixed with other types of systems. These systems, however, have more limited utility than HARD systems. Given their lack of deadline support, they are risky to use for industrial control and Robotics. They are helpful in Multimedia, scientific projects etc.