EPEL are Extra Packages for Enterprise Linux.
For EPEL 6 and 7, just execute
yum install epel-release
EPEL 4 and 5 are no longer supported.
An important step after installing the epel repo is to install YUM-Priorities. Do this step before installing any other packages.
yum install yum-priorities -y
Afterwards check if they are really enabled
[main] enabled = 1
Then take a look at CentOS-Base.repo
and give the sources
[extras] a priority of 1 and
[contrib] a priority of 2.
and give the sources a priority of 10
Gratz! You have succesfully installed the EPEL repo.
yum list | grep epel