The term "Real-Time Operating System" REFERS to a system that is subjected to real-time constraints, i.e., the response must be guaranteed within a certain time restriction or the system must satisfy a specific deadline. 

For instance, a flight control system, real-time monitoring, and so on. Real-time systems with timing constraints include the FOLLOWING types:

• Hard Real-Time Operating System — A system of this type will NEVER miss a deadline. The implications of missing the deadline might be disastrous. If tardiness increases, the utility of a hard real-time system's result declines abruptly, and it may even become negative. Tardiness refers to how late a real-time system does a task in COMPARISON to its deadline. As an example, consider a flight controller system.
• Soft Real-Time Operating System — This type of system can occasionally miss its deadline with a low likelihood. There are no dire implications if you miss the deadline. With the increasing delay, the utility of a soft real-time system's result rapidly reduces. Switches on PHONES are examples of Soft Real-Time Operating Systems.