Uses
Linux is most popular used as a server operating system, and in embeddable systems. The stable, customizable, and free nature of Linux make it an ideal choice often in these cases. Many web servers run on Linux, with a LAMP (Linux-Apache-MySQL-PHP) setup being the most common.
Distributions
There are many distributions and variations of the Linux Kernel. Many of the distributions are customized to specific applications and are often made to have a more familiar UI. Debian, Ubuntu, Red Hat, Gentoo, and SUSE are five of the most common distributions of the Linux Kernel.