Linux is the first truly free Unix-like operating system. The underlying GNU  Project was launched in 1983 by Richard Stallman originally to develop a Unix-compatible operating system called GNU (Gnu’s Not Unix), intended to be entirely free software. Many programs and utilities were contributed by developers around the world, and by 1991 most of the components of the system were ready. Still missing was the kernel.  In 1991, Torvalds was a student at the University of Helsinki in Finland where he had been using Minix, a non-free Unix-like system, and began writing his own kernel. He started by developing device drivers and hard-drive access, and by September had a basic design that he called Version 0.01.  This kernel, which is called Linux, was afterwards combined with the GNU system to produce a complete free operating system.

In October 1991, Torvalds sent a posting to the comp.os.minix newsgroup announcing the release of Version 0.02, a basic version that still needed Minix to operate, but which attracted considerable interest nevertheless. The kernel was then rapidly improved by Torvalds and a growing number of volunteers communicating over the Internet, and by December a functional, stand-alone Unix-like Linux system was released as Version 0.11. The Linux kernel received contributions from thousands of programmers. On 14 March 1994, Linux 1.0.0 was released. The Linux kernel has received contributions from nearly 12,000 programmers from more than 1,200 companies.

Linux kernel has since been ported to more computer hardware platforms than any other operating system. It is a leading operating system on servers and other big iron systems such as mainframe computers and supercomputers. As of November 2014, more than 97% of the world’s 500 fastest supercomputers run some variant of Linux, including the top 80.

Linux also runs on embedded systems (devices where the operating system is typically built into the firmware and highly tailored to the system) such as mobile phones, tablet computers, network routers, building automation controls, televisions and video game consoles; the Android system in wide use on mobile devices is built on the Linux kernel.

Reference:

http://en.wikipedia.org/wiki/Linux

http://en.wikipedia.org/wiki/Linux_kernel

http://www.livinginternet.com/i/iw_unix_gnulinux.htm