After Redhat 9 Redhat split their development efforts into 2 products. Redhat Enterprise Linux (RHEL) is the one that is produced by Redhat itself and is aimed mainly at servers or business desktops. It has long release cycles but to get updates and even to download it you will need to buy an RHN subscription (expensive). Luckily because all the software they use to make RHEL is open source they have released the source packages for it on their ftp site. There are now several RHEL 'clone' distros which consist of the source packages taken from Redhat's FTP site and recompiled. One of the more popular ones of these is Centos (www.centos.org
Fedora is produced by the community and is really a seperate community project from Redhat. They use it as a testbed for new technologies (eg. SELinux, Xen) before they go into RHEL.