RHEL as of version 3 went commercial and required that anyone using it had a support license. Due to GNU & etc Red Hat still had to make the source freely available. So a number of new Linux flavors were started based on RHEL. All they needed to do was remove all of the Red Hat trademarks & logos, basically.
CentOS is basically a clone of RHEL that is distributed freely. CentOS aims to be 100% binary compatible with RHEL. So when running CentOS you are pretty much running RHEL. Only differences that I've noticed is with the up2date feature of RHEL, which requires a support license to access. CentOS uses a different program that does basically the same exact thing.
As far as I can tell, CentOS is quite secure. Obviously it needs all software patched and updated pretty much the same as any other Linux flavor. CentOS 4 / RHEL 4 has selinux, I believe as part of the linux kernel, which can be used to lock down the system even more.
Most hosting companies that I've seen usually offer RHEL or CentOS. I see that as a vote of confidence for them both.
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