What is Linux?

Linux is a multi-user, multi-tasking, UNIX-like operating system. It's the brainchild of Linus Torvalds, a Finnish student now working for Transmeta corporation.

Linux is POSIX-compliant; however, to my knowledge it has not been certified as such.

Linux comes in many minor variations, and is normally delivered with full source code to the entire operating system. If the source does not come with Linux, the source is readily available.

Linux runs on a large number of platforms, including Intel-based PCs (and workalikes), Digital Alpha, Macintosh Power PCs, Sun processors, the Amiga, and others; more platforms are being added all the time.

More on this: http://www.linux.org.