CentOS (Community ENTerprise Operating System) is a freely-available Linux distribution that is based on Red Hat Enterprise Linux (RHEL), it’s the Red Hat’s commercial Linux product. Differently with previous completely free Linux product, RHEL is aimed for commercial corporate solution which maintained and supported by Red Hat.

RHEL is mostly composed of free and open source software and is made available in a usable, binary format on CD-ROM or DVD-ROM only to RHEL paying subscribers. Then as required, Red Hat should release all source code for the product publicly under the terms of the GNU General Public License and other licenses.

CentOS developers then use the released source code to create a final product that is very similar to RHEL and freely available for download and use by the public, but not maintained or supported by Red Hat. CentOS rebuild project strives to be 100% binary compatible with the RHEL product and within its mainline and updates, not to vary from that goal.

There are other distributions derived from RHEL’s source as well, but they have not attained the surrounding community that CentOS has built. CentOS is generally the one most current with Red Hat’s changes.

CentOS mostly installed on server provided by value web hosting companies. Usually CentOS is combined with the cPanel Control Panel. CentOS can be used as an X Window System-based desktop but, like RHEL, but obviously is targeted primarily at the server market.

CentOS’ preferred software updating tool is based on yum, although support for use of an uptodate variant exists. Each may be used to download and install both additional packages and their dependencies, and also to obtain and apply periodic and special (security) updates from repositories on the CentOS Mirror Network.

CentOS version numbers have two parts, a major version and a minor version. The major version corresponds to the version of Red Hat Enterprise Linux from which the source packages used to build CentOS are taken. The minor version corresponds to the update set of that Red Hat Enterprise Linux version from which the source packages used to build CentOS are taken. For example, CentOS 4.4 is built from the source packages from Red Hat Enterprise Linux 4 update 4. Since mid 2006 though, starting with RHEL 4.4, Red Hat has adopted a versioning convention identical to that of CentOS, e.g., RHEL 4.5 or RHEL 3.9.