CentOS, also known as community enterprise operating system, is a recompiled version of Red Hat enterprise Linux (hereinafter referred to as RHEL), the leader of enterprise Linux distribution. RHEL is a Linux distribution adopted by many enterprises, but if you want to get RedHat's service and technical support, users must pay Red Hat. CentOS developers used the source code of Red Hat Linux to create a Linux that is almost the same as RHEL. But all the trademarks related to RedHat were removed, because RedHat didn't allow them to do so. CentOS is free. You can use it to build an enterprise-level Linux system environment like RHEL, but you don't have to pay any fees to RedHat. At present, the technical support of CentOS is mainly provided through official mailing lists, forums and chat rooms in the community.