Linux is freely-distributable open source operating system that runs on a number of hardware platforms. The Linux kernel was developed mainly by Linus Torvalds.
Because it's free, and because it runs on many platforms, including PCs and Macintoshes, Linux has become an extremely popular alternative to proprietary operating systems.
Linux is in the Unix-like family of operating systems, with several major sub-categories including System V, BSD, and GNU/Linux. The name "Unix" is a trademark of The Open Group, but is commonly used to refer to the large set of operating systems which resemble the original Unix.
Unix systems run on a wide variety of machine architectures. Unix systems are used heavily as server systems in business, as well as workstations in academic and engineering environments.
Linux and BSD are increasingly popular, and have made inroads on the desktop market as well.