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RHEL giving good support and updates. Example: If any of the package making OS level dependency with your production environment, support will address the issue if isssue is relevant they will check with internal team and will resolve the issue.
Development purpose and non critical application not depend with OS level, we can use CentOS operating system. I'm regularly using CentOS version for all development activities and will get the good yum repositories for the update also.
I will try to answer this Q from another point of view (unavailability products ) CentOS "As guys said in the previous answers" I personally used to use this distro and deal with it same as RHEL till I had to do some things regarding RHEV and here differences came as such solutions from RHEL not yet available as a community distros so if you wanna to deal with RHEV or Satellite servers etc ...., and may be much more you only have to use RHEL products even as a revolution versions and not waste your time searching for such in the CentOS website
Firstly, CentOS now belongs to Red Hat. :)
Red Hat offers specific Red Hat Linux distributions targeting specific consumer segments. For example, for enterprises, they have RHEL (Red Hat Enterprise Linux). RHEL must be purchased before using it. RHEL is entirely based on free/open source software. But, the way it is compiled and distributed - here lies the big difference. Red Hat thoroughly checks the packages of RHEL. Further, only proven software applications/packages gets into RHEL. Red Hat compiles RHEL packages in a unique way, which can't be mimicked by others. This makes it distinct among its competitors.
CentOS is a community operating system offering "RHEL" like for free of cost. The upstream base for CentOS is RHEL. So, whatever changes RHEL does, CentOS incorporates the changes as well. CentOS is basically used by SMB (small and medium businesses). For such businesses, the difference between RHEL and CentOS is almost zero. In fact, graphics are the only difference. But for big enterprises, the differences vary. Red Hat offers excellent paid support, whereas CentOS doesn't. There are many such differences.
Recently, CentOS has become part of Red Hat. To understand more what CentOS has to offer now, please visit http://www.centos.org
Both are Server level distributions. CentOS is open source means their repositories are accessible while Redhat is not open source. You must pay inorder to access its all repositories and for support. CentOS has no support as you yourself have to do so