An operating system is a system software which acts a bridge between the computer system and the user. The operating system hides all the complicated commands and transitions that occur in a system from the novice user there by providing a very user friendly approach in which the user is least bothered about what goes on inside the system.
examples of some mostly used OS are : Microsoft Windows and Linux Variants such as fedora.
I was just curious of how many OS would be existing in the world. Going through the [linux.org](http://www.linux.org) searching for the total number of linux operating systems available till today i got an amazing 364 distributions. check [here](http://www.linux.org/dist/list.html) !!!Well even Microsoft the arch rival of the free unix based OS ahs its own version of Linux mslinux..that clearly justifies how microsoft evaluates their own software with their nearest competants to have a better and more user friendly OS.
With the rapidly emerging free OSes like fedora and other versions of unix have just increased the options for choosing the right OS. Choosing a operating system clearly depends upon how comfortable one feels with a particular OS and if that OS satisfies his needs. However recently the grip of microsoft has been threatened by the unix counterparts. Microsoft's OSes have been the best choice of most people because of the varieties of application softwares available for use in the Windows OS..but the scene is going to be reversed very soon with almost all the application softwares being developed by the free software community for the linux OS. Mozilla web browser ([here](http://mozilla.org)) has been a grand success. Well time is going to tell us if Linux can take over the OS market beating the Microsoft's Windows series...