The software that provides an interface between a user or application and the computer hardware. Also, the most important program that runs on a computer. Every general-purpose computer must have an operating system to run other programs. Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers. Allows two or more users to run programs at the same time.
Some operating systems permit hundreds or even thousands of concurrent users.
Allows more than one program to run concurrently. Responds to input instantly. The application programs must be written to run on top of a particular operating system. Your choice of operating system, therefore, determines to a great extent the applications you can run. I nterconnection , an ISO standard for worldwide communications that defines a networking framework for implementing protocols in seven layers.