Linux is distributed by its author only as a kernel. Other people have put together ``distributions'' that can be used. These distributions pair the kernel as released by the author with software, to make a complete working package. Most releases include application programs as well as system software, providing ``one stop shopping'' for Linux.
There are several distributions of Linux, which are available at various sites. Sunsite mirrors almost all the distributions in /pub/Linux/distributions. The most commonly-recommended freely-available distributions are Red Hat, Debian, and Slackware. These are all available for free over the internet, and are also sold on floppies and CD-ROM's.
There are other distributions of Linux as well. Most commercial distributors of Linux advertise in Linux Journal as well.