What does EL5 and EL6 means in term of packages?

EL5 stands for Enterprise Linux 5 (Red Hat Enterprise Linux version or CentOS version) and EL6 accordingly for Enterprise Linux 6.

You can find the current release you are running in:

cat /etc/redhat-release

cat /etc/centos-release

Depending on the version your system is running with you are able to install the packages.

if you still can't find your version with previous answer, you can try this :

cat /etc/*-release

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