Linux is an operating system kernel that is functionally very similar to UNIX, and supports a wide range of features, many of which are found elsewhere only on powerful operating systems. Linux has been ported to an ever-increasing number of computer platforms. Linux is free software. Free in that you receive all of the source code for the whole operating system and are allowed to modify it and redistribute it as you please.
For more information on Linux, read the article "What Is This Thing Called Linux" online in POWERlines Volume 8, No. 3.