CentOS (short for Community ENTerprise Operating System) is a free, open-source, enterprise-class and community-supported computing platform that derived from the same source code with Red Hat Enterprise Linux OS. At present, there have been increasing Internet services or applications around the world that require high stability (including a variety of purposes of servers and business applications) are running on it, rather than RHEL.
CentOS (Community Enterprise Operating System) is a recompiled product based on RHEL's (Red Hat Enterprise Linux) open source. Because CentOS fixed a lot of known bugs on the basis of RHEL, compared with other Linux distributions, its stability is more trustworthy. This is the reason a lot of servers requiring high stability use CentOS to replace the commercial RHEL.
Since Red Hat published its last one (9th) release (Red Hat Linux 9) in 2003, they decided not to provide free technical support & product certification any more, and fully turned to the development of Red Hat Enterprise Linux (RHEL for short). This made some Red Hat rebuilders (open-source Enterprise Editions, such as White Box Enterprise Linux, Tao Linux, X/OS Linux, Scientific Linux) were born at the right moment. The most active one is CentOS (short for Community Enterprise Operating System).