CentOS is a free operating system distribution based upon the Linux kernel. It is derived entirely from the Red Hat Enterprise Linux (RHEL) distribution. CentOS exists to provide a free enterprise class computing platform and strives to maintain 100% binary compatibility with its upstream source, Red Hat. CentOS stands for Community enterprise Operating System.
In July 2010, CentOS overtook Debian to become the most popular Linux distribution for web servers, with almost 30% of all Linux web servers using it. But in January 2012, it lost that position to Debian once again.
Read more about CentOS: Structure, Versioning Scheme, Release History, Architectures, Bootable Media Version, Tao Linux, 2009 Organizational Difficulties