Redhat Enterprise LinuxEPEL (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 Red Hat Enterprise Linux (RHEL) and its compatible spinoffs, such as CentOS.

The EPEL project provides add-on packages for Red Hat Enterprise Linux 5.x and 4.x releases and compatible derivatives. EPEL is purely a complimentary repository that provides add-on packages, and will not replace packages provided within RHEL. In other words, if you're looking for the latest build of PHP, you'll need to look elsewhere!

You can configure yum to use the EPEL respository using:
[[email protected] tmp]# su -c 'rpm -Uvh http://download.fedora.redhat.com/pub/epel/5/i386/epel-release-5-2.noarch.rpm'
Retrieving http://download.fedora.redhat.com/pub/epel/5/i386/epel-release-5-2.noarch.rpm
warning: /var/tmp/rpm-xfer.AbHudm: Header V3 DSA signature: NOKEY, key ID 217521f6
Preparing... ########################################### [100%]
1:epel-release ########################################### [100%]

You should then be able to use yum as normal, but have access to additional packages. You can identify where packages are coming from by looking at the output. For example:
[[email protected] tmp]# yum install bonnie++
Loading "rhnplugin" plugin
Loading "installonlyn" plugin
This system is not registered with RHN.
RHN support will be disabled.
Setting up Install Process
Setting up repositories
Reading repository metadata in from local files
Parsing package install arguments
Resolving Dependencies
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for bonnie++ to pack into transaction set.
bonnie++-1.03a-6.el5.i386 100% |=========================| 4.5 kB 00:00
---> Package bonnie++.i386 0:1.03a-6.el5 set to be updated
--> Running transaction check
Dependencies Resolved

Package Arch Version Repository Size

Installing:
bonnie++ i386 1.03a-6.el5 epel 42 k
Transaction Summary

Install 1 Package(s)
Update 0 Package(s)
Remove 0 Package(s)
Total download size: 42 k
Is this ok [yN]:

EPEL packages are located at their master mirror, or you can check the mirror list. You can find help or discuss issues in the epel-devel-list or report issues against EPEL to via bugzilla.