In order to separate RedHatEnterpriseLinux from the community product, RedHatLinux has become a community project (the FedoraProject), and the releases are now known by the name FedoraCore. FC 4 was released on 13th June 2005, FC 3 was released in November 2004, FC 2 was released in May 2004. As of the 6 release the name is going back to just plain Fedora. Much of the reaon for this is that many of the repositories are going to be included in the project.
Fedora Core is defined as "the distribution: the package set that is included on the set of ISO images and directory tree blessed by the steering committee and released as FedoraCore. The steering committee sets policies for FedoraCore, and provides the infrastructure to build it. RedHat Network carries the core package set."
In order to distinguish FC from RedHatEnterpriseLinux, you will get "test" releases of Fedora Core rather than betas, and "updates" rather than errata.