Linux is a Unix-like operating system for personal computers, mobile devices, mainframes, supercomputers, embedded devices, such as routers, and wireless access points. Linux is free and open-source. It is also used in the Android operating system for tablet computers, smartphones, and smartwatches. It is released under the GNU General Public License version 2 (GPLv2). The Linux kernel is a monolithic kernel, supporting true preemptive multitasking.
The Linux kernel was created in 1991 by Linus Torvalds for his personal computer. Linux quickly attracted developers and users around the world, and is now developed by contributors worldwide. The Linux kernel API is very stable and does not break userspace programs.
A great advantage of Linux is that programmers are able to use the Linux Kernel to design their own custom operating systems, like CentOS, Red Hat Enterprise Linux, Debian, Knoppix, Ubuntu, and Fedora.
The Linux Security Module (LSM) provides a mechanism for security checks to be hooked by new kernel extensions. The primary users of the LSM interface are Mandatory Access Control (MAC). Examples include SELinux, Smack, Tomoyo, and AppArmor.