Linux is the name of a Unix like Operating System, originally written by Linus Torvalds, who wrote the Linux Kernel. Often called GNU/Linux to denote the kernel and related tools.
There are several distributions of Linux, that include all the related tools one would need for desktop and server installations. The most common user interfaces for Linux are Gnome and KDE, but Linux can also be used from a command line interface, or headless through a COM or ssh/telnet interface.
There are also several specialized distributions, for example firewall packages where a system can be turned into a network router and firewall, or for embedded devices. Originally designed for the i386 computer, Linux now runs on just about all common processors. Linux is often used as a server OS, even though the desktop support is excellent.