Linux is an open source operating system. Although traditionally used on servers, Linux has recently been gaining acceptance on home desktop computers. Linux is often perceived as an 'alternative' operating system to Microsoft Windows or Apple OS-X due to its non-commercial nature. Linux is developed by a team of thousands of volunteers worldwide, and most distributions are freely distributed.
The name "Linux" technically refers only to the kernel of the operating system. Distributors take this kernel and add their own packages and programs to it, then distribute it as a distro. However, popular use refers to any Linux distro simply as "Linux", just as Windows XP and Windows 98 are both referred to simply as "Windows". It is the popular definition, and not the technical definition, that you will see referred to on this website.
Popular Linux distros include Red Hat, Fedora, SUSE, Debian, Ubuntu, Slackware, Gentoo, and Mandriva. There are literally thousands more. Note that Linux's GPL licence makes the forking of a distro possible- that is why Red Hat and Fedora are usually associated with one another. The same company distributes both of them, however to different users for different purposes. Another similar situation is seen between Debian and Ubuntu. Ubuntu is a Debian fork. That means that Ubuntu's developers adopt a stable Debian release and modify it to create Ubuntu. This practice is legal and encouraged in Linux circles.
Linux Open Source Open_Source Software Technology Questions