|What is Linux|
Linux in narrower sense ...
... is "only" the kernel of a UNIX like Operating System, developped by Linus Thorvalds and hunderts of volentiers. The kernel of it's own is not yet really useful. In order to get a complete Operating System you need a lot of additional programs (so called system - binaries). As most of those programs originate from GNU (resp. the FSF) one often talks of GNU/Linux Systems.
Linux in wider sense ...
... is not a single Operating System, but a collective term for all Operating Systems using the Linux Kernel. Most ( quite likely all ) of those Systeme get the most of their Systembinaries from GNU, and are therefor GNU/Linux Systems.
GNU/Linux Distributions are available for most Processor Architectures:
The IBM - PC , the Apple , the Amiga, for DEC-Alpha, MIPS and SUN Workstations, even for Mini PCs like the Palm Pilot. (these are only some examples)
There are GNU/Linux Distributiones in many shades:
starting with Mini Distributions that fit on one single diskette:
TomsRTBT (one single diskette) specially tailored for data resque and network diagnosis.
continuing with big Distributions, delivered on several CD-ROMs:
Debian GNU/Linux - a mature Distribution, contains free Software only
up to Distributiones for Supercomputers (computer farms so called Clusters) :