GNU/Linux has many important features that other desktop operating systems lack. These were mentioned above, and are discussed briefly in turn here. It should be mentioned that ``Linux'' strictly refers only to the operating system itself (sometimes called the kernel). When one ``buys Linux,'' one is really purchasing a distribution, which includes a kernel, but also the hundreds of programs needed to run a computer system, as well as application software. There are literally dozens of distributions, but the most popular ones in the U.S. are Red Hat, Caldera, Debian, and Slackware. In Europe, S.u.S.E [sic] and Mandrake (a derivative of Red Hat) are popular.