Linux is a freely available, open-source computer operating system that supports a large variety of computing devices. Among these are: desktops, laptops, mainframes, mini-computers, PDAs and other embedded devices.
The Linux Kernel was originally developed by Linus Torvalds based on the Minix operating system, it was originally designed to emulate the Unix operating system. Additional software, written by the Gnu Software Foundation provided ports of many popular Unix commands.
Today, Linux is used just as much as Unix is and can be considered an operating system in it's own right. Indeed, Linux software developers have developed new technologies specific to Linux such as new filesystems and process management that make it an entirely new O/S.