An operating system is program that makes a user easily interact with a computer or in other words through operating system a user can easily interact with the computer software's and it's hardware easily and effectively

There is two type of operating systems:

1) Command-line operating system

Example DOS

In this kind of operating system, each and every task is done by a set of command and computer answers accordingly. There is different type of command assign for different task. That's made it complex for user to use and understand.

2) With a graphical user interface (GUI) operating system
Example windows

This operating system is more users friendly and supported by the majority of users only due to its easily understandable nature. In this type of O/S we directly interact with the computer through a graphical interface with pictures and buttons by using the mouse and keyboard. At the end we can say that GUI is simpler then any command based operating system.

What is Linux OS?

Linux is a freely available open source operating system under GNU public license. It is freely available to download from the Internet. You can download it for both CD�S and DVD�s.

Linux is developed by hundreds of developer�s worldwide and many people are still working on it for developing Linux more users Friendly. You can also make your own Linux by using the source code of available Linux it is free.

Linux is extremely powerful and is very stable operating system for hosting Web servers. This powerful operating system can work on different hardware platforms for example Intel, Sparc�, PowerPC, and Alpha Processors.

Basically Linux is an improved version of UNIX and today many distribution of Linux are available in the market. The major names are Red Hat Linux, Fedora, Suse, and Mandrake Linux. These versions required to be installed on the System.
 
But there are also some Linux operating systems available that can be directly boot from the CD and we can work on it without installing it explicitly on the system, for example KNOPPIX and many more.

Source: http://relator.in/linux/os.shtml.