Linux is a Unix-like computer operating system (in a similar manner that a Mac is a Unix-like computer operating system) assembled under the model of free and open source software development and distribution. The defining component of Linux is the Linux kernel, an operating system kernel first released 5 October 1991 by Linus Torvalds. Since the C compiler that builds Linux and the main supporting user space system tools and libraries originated in the GNU Project, initiated in 1983 by Richard Stallman, the Free Software Foundation prefers the name GNU/Linux.
There are many variants or distributions of GNU/Linux, the main distribution that Sector Alpha uses on it's servers are Ubuntu.
Bloodlust80 has kindly put together a basic tutorial on how to use Unix/Linux. If you are lucky enough to have a SASHA or ALESSA account, you will need to learn how to use Unix in order to operate it.