The Red Hat Linux product doesn't actually exist any longer. The original marketing model was to make Red Hat Linux free, then charge for premium support. The problem was that since support packages cost thousands and the actual product was free, companies hired IT professionals who could take care of Red Hat Linux without a support contract with Red Hat. So to improve on Red Hat's marketing of the premium product, Red Hat Linux was split into two products; Red Hat Enterprise Linux (RHEL) and Fedora.
Red Hat Enterprise Linux is designed to be stable and suitable for a corporate environment. While it doesn't necessarily have the latest releases of applications, the applications that come with Red Hat Enterprise Linux are heavily tested and believed to be stable enough for Red Hat's corporate customers. Red Hat Enterprise Linux costs thousands.
Fedora is considered to be experimental, since it has the latest applications included. There's nothing inherently unstable about Fedora, and I've even used Fedora as a production environment. However Red Hat doesn't consider Fedora to be stable enough for it's corporate customers. Fedora is free.
CentOS is an open source (free) product based on Red Hat Enterprise Linux source files, which are required to be in the public domain according to the Linux license. Most of us use CentOS because of its stability and similarity to Red Hat Enterprise Linux. I read somewhere that more than 2/3rds of all web server worldwide currently use CentOS.
I recommend CentOS. For the time being I recommend CentOS 5.x 32-bit
, since Kloxo still doesn't work well with CentOS 64-bit or CentOS 6. Here are the North American mirrors for CentOS.