Let's just say it, CentOS is a RedHat rip off!
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.
If you want RedHat, but do not want to buy a license (which is why we do not offer RedHat), consider CentOS.
CentOS uses yum with RPMs for its package system. The system is pretty basic with not much new software. If you are looking for things like PHP5, don't use CentOS. We use CentOS for our mailserver, based on qmail-toaster. We have this image available to you as well.
We recommend CentOS only for those that are familiar with RedHat and want a system that looks like it. Otherwise, if you want to stay RedHat-like, choose a Fedora Core. Or go with our personal favorite: debian based systems.
Check out the other images we have.