If you are deciding between RHEL or CentOS then the difference is a clear. CentOS is the freeware alternative to RHEL. While Red Hat Enterprise Linux (RHEL) will require a license from Red Hat to use. RHEL also comes with technical support for the duration of whatever license you purchase.
With CentOS you are responsible for all technical issues and updating the operating system. While with RHEL you can contact Red Hat for technical support if your license is still valid.
CentOS: Free, you are responsible for updating and administrating the operating system, community driven
RHEL: Costs money, can contact Red Hat for support, corporate backed support
If you are running a hobby server or a server that doesn’t require a corporation to provide support and are familiar with Linux systems then go with CentOS. If you are a company that requires a company be there to provide support when needed then go with RHEL.