This is a custom, addon repository, such as custom-el5-buildsys. It contains addon packages which may override packages from the base operating system or other third party addon repositories. In the case of custom-el5-buildsys, such packages may include yum (from 3.2.22 to 3.2.24), rpm, koji, and so forth. Each of the packages overriden through this repository should have its own explanatory paragraph or page.
This is the simplest of repositories by Ergo Project. This is an addon repository which does not override any package.
This is a feature addon repository, that is definitely going to override packages provided by the base operating system or other third party addon repositories. For instance, feature-el5-ruby-1.8.6 is one of such repositories, providing a ruby-1.8.6 stack for the Enterprise Linux 5 branch of operating systems.
The version number of the Ruby stack is included, to be able to also supply a 1.8.7, 1.8.7-ee and 1.9.1 stack for Enterprise Linux 5 (and later Fedora 11, 12, 13 and Enterprise Linux 6).
Note that feature repositories do not necessarily conform with update and upgrade policies of the upstream branch of operating systems.