Hi Jose,

What is libX11 ?

X11 is the basis for the GUI in most Linux systems (in the future people are starting to look at a display technology called Wayland, but that is still some way off from being the default tech for display in Linux)

libx11 provides the shared libraries that the client applications need to display data on your desktop.

Wikipedia has a good introduction to X11 and it's history at http://en.wikipedia.org/wiki/X_Window_System

Yum, Repos, and Third Part Software

Repositories (often abbreviated to repos) are software sources. You can set up your own repos, connect to third party repos (more on those in a moment). Red Hat Network also supplies software through a repository, but we have some different terminology and plugins to yum to enable those.

A quick introduction to yum, is What is yum and how to use it? which covers those concepts in more detail.

There are other sources for software that will run on RHEL besides Red Hat, the EPEL repo being the most visible in the enterprise world.

It is important to understand that the software in these repos is unsupported by Red Hat. In very rare cases GSS (Global Support Services) may ask that EPEL software be removed to diagnose a problem. This isn't the default position, but in some instances we need remove applications we didn't supply to rule them out as cause.

You can find more detail about EPEL in Does Red Hat Support The EPEL Repository?.

Adding a repo is discussed in the Deployment Guide.

HTH, please let us know if you need more information
Mark