The problem with Fedora is that it is bleeding edge - and can be a bit unstable. So they let things go into Fedora, wait a year or two for it to stabilise and g

The problem with Fedora is that it is bleeding edge - and can be a bit unstable. So they let things go into Fedora, wait a year or two for it to stabilise and get the bugs worked out, and then it goes into RedHat/CentOS. That is, at least for major versions of software - they constantly update packages with security fixes that might come out, and continue this for years. This means even backporting security fixes from newer releases. So you use CentOS when you don't mind packages being a little old, but want a very solid system with long term security updates (and there are the Software Collections coming out which let you get newer versions of certain things - like Postgres which is still 8.4 in the stock install)