A version of the Linux operating system and related components. A Linux distribution includes the open source Linux kernel, software modules from the GNU Project, an installation program, window manager (user interface), utilities, applications and documentation. Some distributions contain a thousand or more files. See GNU/Linux. Linux distributors may offer both paid and free versions, whereby the free version comprises an older kernel, fewer utilities and no support, other than written help and forums on their website. The paid version includes the latest modifications, a multitude of related software and a support program. For example, Red Hat's Fedora is free of charge but offered without support (see Fedora). Why Is It Called a Distribution? Unlike Windows and Mac, a Linux distribution from two different vendors is not like the next version of Windows or Mac. Each Linux "distro" has components that may differ slightly or greatly from another Linux distribution. There are hundreds of Linux distributions. For the hundred most popular ranked by hits per day, visit www.distrowatch.com. See Linux, GNU, GNU/Linux, arch, embedded Linux and open source.