What's the difference between RHEL and CentOS?

1. Fedora is the main project, and it’s a communitity-based, free distro focused on quick releases of new features and functionality.
2. Redhat is the corporate version based on the progress of that project, and it has slower releases, comes with support, and isn’t free.
3. CentOS is basically the community version of Redhat. So it’s pretty much identical, but it is free and support comes from the community as opposed to Redhat itself.

There's no functional difference. 
 CentOS is RHEL relabeled to remove the Red Hat identity. 
Red Hat isn't going to support it, but you do get the benefit of all the work they put in on integration of the distribution,