You may be aware that RedHat began charging for their latest versions, post RedHat 9, calling it RedHat Enterprise Linux (RHEL). CentOS is basically RedHat Linux which remains free. CentOS (Community ENTerprise Operating System) is an open source free GNU Linux distribution which aims to be 100% compatible with and based on Red Hat Enterprise Linux.
 

[INDENT]Purpose of CentOS (from centos.org)
 
CentOS exists to provide a free enterprise class computing platform to anyone who wishes to use it. CentOS 2, 3, and 4 are built from publically available open source SRPMS provided by a prominent North American Enterprise Linux vendor. CentOS conforms fully with the upstream vendors redistribution policies and aims to be 100% binary compatible. (CentOS mainly changes packages to remove upstream vendor branding and artwork.). CentOS is designed for people who need an enterprise class OS without the cost or support of the prominent North American Enterprise Linux vendor.
 
Neither the CentOS Project (we who build CentOS) nor any version of CentOS is affiliated with, produced by, or supported by the prominent North American Enterprise Linux vendor. Neither does our software contain the upstream vendor\'s product ... although it is built from the same open source SRPMS as the upstream enterprise products.
 

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