If you are new to server terminology, you may not fully understand what Linux is. While it is commonly used to refer to an operating system, there is actually no operating system called Linux.
Linux is actually the kernel of an operating system. A kernel is the core that interacts with hardware, contains drivers, and is generally necessary to make the computer work. Since Linux is only the kernel, it requires other components to make a full operating system. Although this is generally referred to as GNU/Linux, most Linux distributions also include other tools beyond the GNU system.
Any operating system that uses the Linux kernel is generally considered a Linux distribution, although some would limit the term “distribution” to those that include GNU. Others like Android, would be considered Linux-based OSes but not distributions. Since there is no established regulations for what is considered a Linux distribution, opinions vary on what is and what is not. It is also important to note that the name “Linux” is a trademark and not every Linux distribution includes the word “Linux” in its name. Ubuntu, for example, is the name of one of the most popular Linux-based OSes that is also a Linux distribution.
Since a Linux distributor can include anything they want in their version of Linux, some have catered them specifically for servers. These include Red Hat Enterprise Linux, SUSE Linux Enterprise Server, and the increasingly popular free server distribution CentOS. Other distributions are not specifically made for servers but generally work very well in server environments. These include Debian, Slackware, and Gentoo.