We are now all aware that an operating system is the program that is initially loaded into the computer by a boot program. It manages all the other programs in a computer, called applications. These application programs make use of the operating system by making requests for services through a defined application program interface (API).

What does one mean by open source? Open source refers to programs where source code is made available to the public to use and also modify from the original design, free. Open source code is usually produced as a joint effort in which programmers' progress upon the code and share changes with the interested parties. These open source codes were born in technological community as an answer to proprietary software of ownership of corporations.

A certificate was given by the Open Source Initiative (OSI), indicating that source code of a computer program is accessible free by general public. Open Source Initiative has set down criteria's for a product to be accepted as 'OSI Certified'. These criteria's are as follows: The license holder or the author of source code may not collect royalties on the circulation of the program. The source code of the distributed program must be made available to the user. The license holder must permit alterations and derivations of the original work under the program's original name. No group, field or person can be denied rights to use to the program. The rights of the program shouldn't depend on the fact that the program is being part of a particular software distribution. The licensed software cannot put limitations on other software that is circulated with it.

Many open source operating systems are available for users. Linux is only one of the most common open source operating system available for use and modification. There are many open source operating systems available, namely, 'Kubuntu Linux', 'Ubuntu Linux', 'Suse Linux', 'Mandriva Linux', 'Debian Linux', 'Elive Linux', 'Austrumiis', 'Ubuntu Studio', 'JAD Studio', 'Fedora Linux', 'Sabayon Linux', 'Mepis Linux', 'Pardus', 'Slackware Linux', 'Dynebolic Linux', 'Dream Linux', 'Free BSD Unix', 'PC BSD Unix', 'DesktopBSD Unix', 'ReactOS', 'GNU-Darwin Linux', 'Haiku', 'AROS', 'Menuet', 'Minux3', 'Syllable', 'Geexbox Linux', 'XBox Linux', 'FreeVMS', 'JNode', 'BeleniX' (an OpenSolaris distribution), 'FreeDOS', etc.

Linux is the probably the best open source operating system available. FreeBSD is second only to Linux among open source operating systems in popularity and community size. The advantages of using open source operating systems is that it has a lower cost of ownership, reduced dependence on software vendors, easier to customize, higher level of security, etc.