Linux is a Unix-like computer Operating System (or OS) that uses the Linux kernel which is developed under open-source software. Linux started out as a personal computer system used by individuals, and has since gained the support of several large corporations, such as Sun Microsystems, HP and IBM. It is now used mostly as a server operating system, with some large organizations using an enterprise version for desktops.
The development of Linux is one of the most prominent examples of free and open source software collaboration: the underlying source code may be used, modified, and distributed; commercially or non-commercially, by anyone under licenses such as the GNU General Public License. Typically Linux is packaged in a format known as a Linux distribution for desktop and server use. Some popular mainstream Linux distributions include Debian (and its derivatives such as Ubuntu), Fedora and openSUSE. Linux distributions include the Linux kernel, supporting utilities and libraries and usually a large amount of application software to fulfill the distribution’s intended use. 
Currently Linux DOES NOT meet the requirements for National Industry Security Program Operating Manual (NISPOM) Chapter 8. NISPOM is a computer security requirement developed by the Department of Defense Department of Energy and published by the Defense Security Service (DSS) which US defense contractors are required to meet when processing classified data on computers in a classified environment.
AcqLinks and References:
-  Website: Wikipedia – Linux Operating System
- Website: The Linux Kernel Archives