A Distro (short for distribution) is a version of the Linux/Unix operating system bundled together. Which Distro the system administrator selects is largely dependent on the data center hosting the server and what they provide and support. This page provides some general info about the the currently supported distros and gives those who have a choice some more information to help make a decision.
Red Hat Enterprise Linux, RHEL is a commercial product that require a paid license from Red Hat
"CentOS is an Enterprise-class Linux Distribution derived from sources freely provided to the public by a prominent North American Enterprise Linux vendor. CentOS conforms fully with the upstream vendors redistribution policy and aims to be 100% binary compatible. (CentOS mainly changes packages to remove upstream vendor branding and artwork.) CentOS is free." (from the CentOS website).