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:
su -c 'rpm -Uvh http://mirror.switch.ch/ftp/mirror/epel/5/i386/epel-release-5-4.noarch.rpm' ... su -c 'yum install puppet'
su -c 'rpm -Uvh http://download.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm' ... su -c 'yum install puppet'
sudo yum install epel-release