Red Hat Enterprise Linux or CentOS
Red Hat Enterprise Linux or RHEL is a commercially supported distro
http://www.redhat.com/ that has been around since 1995.
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Since it's one of the major commercial distros, other commercial
vendors, HP, IBM, etc, officially support some of their products with
this distro. Since there are numerous distros, they can only afford
to officially support a few.
The packages included in an enterprise Linux version have been time tested (“Soak time”) rather than being the latest versions.
For more information see the link above or see http://en.wikipedia.org/wiki/Red_Hat.
Since RHEL is open source, it is possible to create a distro that contains the same packages but is community supported rather than paying for a support contract from Red Hat. Some companies using Linux want garanteed support contracts and are willing to pay for them.
CentOS http://centos.org/ is a community supported version of RHEL. Since there are no license fees, you can have all the development machines you want without paying any license fees to Red Hat.
One of the project goals of CentOS is to NOT fork from RHEL. Therefore, bugs found in CentOS and any fixes are fed back to Red Hat.