These are the different types of OS.
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.
Multi processing – Supports running a program on more than one CPU.
Multi tasking – Allows more than one program to run concurrently.
Multi threading – Allows different parts of a single program to run concurrently.
Real time – Responds to input instantly. General-purpose operating systems, such as DOS and UNIX, are not real-time.