1. Linux Kernel

The Linux kernel is a free and open-source,[6][7] monolithic, modular,[8] multitasking, Unix-like operating system kernel.[9] It was conceived and created in 1991 by Linus Torvalds[10] for his i386 based PC, and it was soon adopted as the kernel for the GNU Operating System,[11] which was created as open source and free software, and based on UNIX as a by-product of the fallout of the Unix wars.[12] Since then, it has spawned a plethora of operating system distributions, commonly also called Linux.

Linux is deployed on a wide variety of computing systems, such as embedded devices, mobile devices (including its use in the Android operating system), personal computers, servers, mainframes, and supercomputers.[13] It can be tailored for specific architectures and for several usage scenarios using a family of simple commands (that is, without the need of manually editing its source code before compilation);[14][15][16] privileged users can also fine-tune kernel parameters at runtime.[17][18][19] Most of the Linux kernel code is written using the GNU extensions of GCC[20][21] to the standard C programming language and with the use of architecture specific instructions (ISA). This produces a highly optimized executable (vmlinux) with respect to utilization of memory space and task execution times.[22]

Day-to-day development discussions take place on the Linux kernel mailing list (LKML). Changes are tracked using the version control system git, which was created by Torvalds as a bespoke replacement for BitKeeper. Linux as a whole is released under the GNU General Public License version 2 (GPLv2),[23] but it also contains several files under other compatible licenses,[5] and an ad hoc exemption for the user space API header files (UAPI).




Source: Wikipedia.