EPEL stands for Extra Packages for Enterprise Linux. This is a massive collection of packages that are very useful for the building of many modern stacks and include many tools for the running and maintenance of systems.
The homepage of EPEL is here: http://fedoraproject.org/wiki/EPEL.
You can browse the package set using repoview:
Installing on Centos 5.x & 6.x:
For EL5:
su -c 'rpm -Uvh http://mirror.switch.ch/ftp/mirror/epel/5/i386/epel-release-5-4.noarch.rpm' ... su -c 'yum install puppet'
For EL6:
su -c 'rpm -Uvh http://download.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm' ... su -c 'yum install puppet'
For EL7
sudo yum install epel-release