Linux kernel

Linux is an operating system kernel used by a group of Unix-like operating systems. These are known as Linux operating systems.
The Linux kernel is released with the GNU General Public License version 2 (GPLv2) and developed by programmers worldwide.
History.
In April 1991, Linus Torvalds, then 21 years old, started working on some simple ideas for an operating system. Then, on 25 August 1991, Torvalds posted to "comp.os.minix":
After the post, many people made code for the project. By September 1991, Linux version 0.01 was released. It had 10,239 lines of code. In October 1991, Linux version 0.02 was released.
The newsgroup "alt.os.linux" was started, and on January 19, 1992, the first post to alt.os.linux was made. On March 31, 1992, alt.os.linux became "comp.os.linux".
The X Window System was soon moved to Linux. In March 1992, Linux version 0.95 was the first to be capable of running X. This large version number jump (from 0.1x to 0.9x) was because of a feeling that a version 1.0 with no major missing parts would be released soon.
On March 14, 1994, Linux 1.0.0 was released, with 176,250 lines of code. In March 1995, Linux 1.2.0 was released (310,950 lines of code).
Linus decided, on May 9, 1996, to adopt Tux the penguin as the mascot for Linux.
Version 2 of Linux, released on June 9, 1996, was a landmark. Strong development continued:
Currently a new kernel version is released in every 2 3/4 months.
Legal aspects.
Licensing terms.
In the start, Torvalds released Linux under a license which did not allow anyone to sell it. This was soon changed to the GNU General Public License (GPL), during version 0.12. This license allows distribution and sale of modified and unmodified versions of Linux, but requires that all those copies be released under the same license, and be accompanied by the complete corresponding source code.
Torvalds has said licensing Linux under the GPL as the "best thing I ever did".
References.
Sources: