Linux is a free (libre), open source, Unix-like operating system.
Linux was created by Linus Torvalds in 1991 as a hobby project, and quickly caught as the only freely available Unix-like system of that time. Today, it's one of the biggest operating systems, most notably is the Android project which runs on Linux.
The Linux kernel is written mostly in C and is released under the GNU GPL v2 license. It's one of the biggest open source projects to date with 1,500 developers and over 200 companies contributing to the 4.1 version release June 2015 (LWN.net).
Programming under Linux
Linux is a Unix-like mostly POSIX compliant operating system. The GNU Compiler Suite is the standard compiler. It comes with glibc as the standard C library.
Further reading: Advanced Linux Programming by CodeSorcery LLC.
A Linux distribution (or distro) is a bundle of the Linux kernel, a package manager and a set of essential applications. DistroWatch has a comprehensive list of 277 distros as of July 2015. The highest ranked are Linux Mint, Ubuntu and Debian.
48.61% of worldwide device shipments run Android (includes smartphones, tablets, laptops and PCs). (Gartner 2014)
96.20% of the top 500 supercomputers in the world run Linux. (top500.org 2015)