RedHat is the mothership. :-)
RedHat Enterprise Linux (RHEL) is the gold standard edition. Conservative and very stable. You pay for support, and it's VERY good support.
Fedora is the sort of the active development version, many of whose features eventually are incorporated into RedHat. It's for folks who like to be closer to the cutting edge.
CentOS stands for "Community Enterprise Operating System. It is the exact same software as RHEL -- line for line of code and binary -- with only two distinctions:
Caveats: CentOS, like RHEL, is not the cutting edge. It is often a release or two behind the latest-and-greatest advances in applications. Brand new drivers for new peripherals sometimes take longer to get released, because they take lots of time to test and validate. I prefer the stability, and can live without being on the bleeding edge.
Thanks!!!!! For me Fedora looks best. But will use CentOS for stable servers.