One of the first things that confounds new users is that GNU/Linux is not a single operating system. Instead, it's the general name for hundreds of closely related operating systems -- distributions or distros, as they are usually called. All the distros share the Linux kernel and include many of the same utilities and programs, but differ in purposes and the resulting customizations. Although users often choose a major, general purpose distribution, many choose a distribution based on any number of criteria, including support services, efficiency, philosophy, and intended audience.