CentOS is CentOS is an Enterprise-class Linux Distribution derived from sources freely provided to the public by our Upstream OS Provider (UOP). 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.

From the release announcement:
CentOS 5.5 is based on the upstream release EL 5.5 and includes packages from all variants including Server and Client. All upstream repositories have been combined into one, to make it easier for end users to work with. For the complete release notes, refer to http://wiki.centos.org/Manuals/ReleaseNotes/CentOS5.5.

CentOS 5.5 DVD for x86 1 Install DVD for x86 (32 bit)

CentOS 5.5 DVD for x86_64 2 Install DVDs for x86_64 (64 bit)