Linux is an alternative, open source operating system. It is alternative because it runs on the same hardware that the market share leader (Microsoft Windows) runs on, open source because it is not only freely obtained (via various channels such as online downloads, free CDs that come with magazines and books, etc…), but also because its source code (the actual computer instructions that make the software work) is also freely accessible and modifiable.
The fact that Linux is freely available for anyone to obtain and tinker with has been a boon for the adoption and use of Linux with computer enthusiasts, geeks, nerds and the like. These enthusiasts like the fact that the code is right there in front of them for them to change to their needs. A competent programmer, even a hobbyist, can customize Linux to their needs to a much more refined level than they could with MS Windows, Apples Mac OS, and even other more proprietary flavors of UNIX.
There is a very active and thriving developer community surrounding Linux. There are literally thousands of websites out there devoted to the subject matter. All sorts of knowledge bases, forums, chat rooms and message boards are dedicated to the goings on in the Linux and open source community.
Another excellent facet of the Linux community is the fact that the majority of the software applications written for the operating system is also made available with the same open source license, which means that an enthusiast (or a professional for that matter) can build and customize a Linux system that is perfectly tailored to their needs with little to no investment beyond the initial purchase of the hardware.
A sound studio engineer, for example, could build a Linux machine that has been custom tailored to run the effects on a sound board or to function as a recording device for digital audio, while a graphic artist may build a machine that has been built from the ground up to be an efficient design machine.