Operating systems can be classified as follows:

1)Multi-user: Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users. (2)Multiprocessing : Supports running a program on more than one CPU.(3)Multitasking : Allows more than one program to run concurrently.(4)Multithreading : Allows different parts of a single program to run concurrently.(5)Real time: Responds to input instantly. General-purpose operating systems, such as DOS and UNIX, are not real-time.