CentOS is an Enterprise-class Linux Distribution derived from sources freely provided to the public by the Upstream OS Provider (UOP)1.
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 a Free Operating System.
CentOS 5.1 is the first update to the CentOS 5 distribution series. It contains a lot of bug fixes, updates and new functionality. We advise you to read the UOP release notes at http://www.centos.org/docs/5/.