How To Enable EPEL Repository on RHEL/CentOS
If you are using CentOS or RHEL, it is strongly recommended that you enable EPEL (Extra Packages for Enterprise Linux) repository on your system. EPEL (Extra Packages for Enterprise Linux) is a volunteer-based community effort from the Fedora project to create a repository of high-quality add-on packages that complement the Fedora-based Red Hat Enterprise Linux (RHEL) and its compatible spinoffs, such as CentOS and Scientific Linux. Once you set up EPEL repository, you can use yum / dnf command to install software packages from EPEL repos.
EPEL provide lots of packages for CentOS / RHEL, It is not part of RedHat or CentOS but is designed to work with these major distributions. Please note that EPEL only provides free and open source software unencumbered by patents or any legal issues. In short you will not find mp3, dvd and music / media player under EPEL. However, you will find many programs related to networking, monitoring, sys admin, programming and so on.
You can get Non-free softwares by install/enable RPM Fusion repository on your Fedora/CentOS/RHEL or other derivative systems.
Now see How to enable it on various versions of RHEL/CentOS :
For RHEL7/CentOS 7
RHEL7/CentOS 7 users can install EPEL repos by running following command..
sudo rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
OR
Simply , by running..
sudo yum install epel-release.noarch
For RHEL6/CentOS 6
sudo rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
For RHEL5/CentOS 5
sudo rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-5.noarch.rpm