The word Linux commonly refers to the increasingly widely-used GNU/Linux computer operating system, based on the "Linux" kernel and hundreds of Free (Libre)/Open Source programs including most crucially the GNU utilities. Individuals and organisations are free to arrange and organise these various tools as they wish, including adding to and removing from them to suit their own needs so there are dozens of different "distributions" of this operating system and application software. All partake of several essential core qualities.
The Free Software Foundation (Source of the GNU programs)
An absolutely fantastic tutorial for Linux, starting from scratch and going all the way to linux guru, is available here. Of course, Internet explorer is incapable of reading it. ;)
The topics at Category: General computing may also be of interest.