For those of you who don't know already, Linux is an operating system (like Windows or Mac OS-X), that is built by the public and it's free.* If you know how to use Windows or OS-X, You can install Linux and be using it with no training in about 30 minutes. It's made entirely of programs donated to the public good and many distributions come with office applications, games, and web tools already installed.
There are hundreds of companies, causes, and individuals who package Linux into a "distribution" consisting of the Linux kernel (the guts of the operating system) and a set of software designed for a specific purpose. For example:
- Easy-to use desktop: Ubuntu
- Corporate web-server: RedHat
- Free (GNU) software purity: Debian or gNewSense
- Minimal hardware: Puppy
- Run from a CD: PCLinuxOS
- Repair your computer after a crash: SystemRescueCD
- To be just like another distribution: CentOS (like RedHat)
- To protect networks: IPCop
- Breaking into other computers: PHLAK (Professional Hackers Linux Assault Kit)
- Christianity, bible study, and parental control: Ubuntu Christian
- Online learning environment, suitable for classroom use: Edubuntu
That's a random sampling taken mostly from Distrowatch. There is a lot of crossover between the goals and included packages of various distributions, but this gives you a sense of what's out there. Because the source code for the entire operating system and most of the software generally distributed with it is free and/or open-source, a programmer can compile their own version and alter it to work with custom hardware such as a new phone, or a robot.
Interestingly, no-one told anyone to create Linux. In 1989, Richard Stallman, a visionary programmer who could be called the world's first software activist, created a new kind of software license - the GNU Public License, GPL, or "copyleft. The GPL guarantees everyone's right to copy, share, modify, and (re-)distribute software. In 1991, A programmer named Linus Torvalds started working on a version of Unix that he could use on his own computer. He published it under the GPL and released it on the web and soon many other programmers (or "hackers") contributed changes and improvements. The work of untold numbers of programmers, writers, graphic artists, testers, and other largely self-selected volunteers, under the GPL and other "open source" licenses, has created Linux in all its flavors - and it continues to evolve.
Who uses Linux? We all do. At least half of all web sites run on Linux servers. Mobile phones such as the Android run Linux. 443 of the top 500 supercomputers run Linux. Increasingly, personal desktop computers run Linux. If you aren't familiar with Linux, I encourage you to get yourself a LiveCD of any distribution and give it a try. Ubuntu is currently the most popular for non-techies.
For your entertainment, here's a brief history of Linux through advertising (and one spoof):
Mac Spoof: Upgrading
I'm a Linux
* Thanks to DigitalMan for helping me come up with a one-line explanation of Linux.