The Debian, Ubuntu, Fedora, CentOS and OpenSUSE Linux distributions are among the many available for free download online as of 2015. The majority of Linux distributions, or distros, are free, and most have large user communities that maintain them or companies sponsoring their maintenance who also provide commercial versions.
Technically, Linux is not an operating system. The Linux kernel, originally written by Linus Torvalds, is open source software that forms the basis for all Linux distributions.The kernel is the base layer of code that communicates down to the hardware and up to software packages that need access to the hardware. A Linux distribution combines the Linux kernel with additional software, including package managers, libraries and system tools, and user-level applications. Although most Linux distros include windowing systems and managers, some lightweight distributions contain only command-line interfaces, or CLI, as their desktop environments.