RedHat started as free linux operating system. On my first server, we used RedHat 9 OS. After version 9, redhat stopped providing free OS, started EHEL. Since it is open source operating system, they have to provide source code, many modified the RHEL source code and distributed as CentOS, Scientific Linux, etc.. RedHat still provide community supported free linux Fedora. RedHat is one of the most successful open source company.