CentOS is a linux distribution that tries to be 100% binary compatible with Red Hat Enterprise Linux.
CentOS takes the freely available RedHat Enterprise Linux sourcecode (not the binaries),
removes trademark and branding information that identiﬁes it as a Red Hat product, then
rebuilds and rebrands that source code as CentOS.
CentOS is designed for people that need an enterprise class Linux distribution without having to pay the subscription costs associated with Red Hat Enterprise Linux. The first CentOS release was in 2004 which means that the CentOS Team is experienced and not likely to disappear overnight.
What is attractive from my perspective is that open-source VoIP packages such as Asterisk, Elastix and OpenSIPS are either based on CentOS or have application packages written for CentOS.
In the future I hope to write some VoIP guides based on these applications running under CentOS.