An operating system is a software, or a series of program which performs various types of functions in order to manage
and organize files,”

Functions of an operating system are

(i) Process management

(ii) Memory management

(ii) File management

(iv) Input/output management

An OS can be classified as follows

Single-user

Allows only one user at a time.
e.g. MS-DOS, Windows 9X.

Multi-user

Allows two or more users to run programs at the same time.

e.g. Unix, Linux, Windows 2000/7

Multi-tasking

Allows more than one program to be run concurrently.

e.g. Linux, Unix, Windows 95

Multi-processing

Supports running a program on more than one CPU.

e.g. Unix, Windows NT/2000.

Real-time

Used for real-time based applications like satellite launch, weather-forecasting etc.

e.g. Lynx, HP-RT