If you want to use Red Hat Linux and don't want their community developed, and bleeding edge, Fedora distribution, it would appear you have to pay a substantial amount for it, primarily for the support contract. But Red Hat Linux uses open source software, so that is not the case. CentOS is a reworking of the Red Hat Enterprise Linux source code to make a totally free version. The reworking mainly involves removing any Red Hat trademarked or copyrighted material, and replacing support web address with the CentOS site. In every other respect bar one this is RHEL. The one difference is that you don't get the support contract, but that doesn't mean you are left hanging in the breeze if something goes wrong, it just means you don't have a dedicated telephone support service.
As with some other Linux distros, CentOS has an active community of developers and users to provide help in their forums, mailing lists and IRC channels. There is also plenty of useful and helpful documentation and regular security updates, once again based on Red Hat's own. Like its progenitor, CentOS is intended for use in commercial environments, where stability is more important than having the latest flash plugin for YouTube. CentOS is suitable for use as an office server or on desktop computers for all users. So if you want an enterprise class distribution without the enterprise class fees, and you are prepared to search forums and ask on IRC if you need support, CentOS is an excellent way of getting top class software at the best possible price.
Note that this is the Live CD version, which means you can test CentOS without having to install an OS on your computer. Just burn the disc, boot and test.
For a commercial installation on a budget, or if you already have experiences sysadmins, CentOS provides unbeatable value and stability.