Linux is an free open source OS using an UNIX base .Linux is a project conceived from a hacker community to promote the free software movement, launched in 1983, in order to create an operating system which can be developped by a community and shared without any restriction.

The most popular are Red Hat, Debian and Slackware (they are the base of hundreds of distributions).
Every Linux distribution has its own features. CentOS is optimized for server and network use , Ubuntu has an intuitive desktop interface, BackTrack has pre-installed testing security softwares... And there is many other Linux distibutions with their own specifities and functionnalities.
But the real Linux Power focus in only one thing : the Console! This is the advantage of an UNIX base. You can do everything you want via this interface. Moreover, most hosting provider will give you a server reachable via this tool. It is like the MS-Dos for Windows.

Axelaris-Hosting proposes hosting solutions like VPS with free Linux distributions as Ubuntu, CentOS, S.U.S.E, Fedora...