From CNET News.com:

The clone is from a project called CentOS–Community Enterprise Operating System–one of several “Red Hat rebuilders” that have partially nullified Red Hat’s business decision in 2003 to stop giving away its supported and certified product for free. CentOS and others–Lineox, White Box Linux, Tao Linux, X/OS Linux and Scientific Linux–all rebuild a copy of Red Hat Enterprise Linux from the source code components Red Hat releases.