Scientific Linux is a distribution put together by Fermilab, CERN, and various other labs and universities around the world. Its primary purpose is to reduce duplicated effort of the labs, and to have a common install base for the various experimenters.
The base Scientific Linux distribution is basically Enterprise Linux, recompiled from source.
The main goal for the base distribution is to have everything compatible with Enterprise Linux, with only a few minor additions or changes. An example of items that were added are Pine, and OpenAFS. If a program runs and is certified on the commercial enterprise linux distribution, then it will run on the corresponding Scientific release.
The secondary goal is to allow easy customization for a site, without disturbing the Scientific Linux base. The various labs are able to add their own modifications to their own site areas. By the magic of scripts, and the anaconda installer, each site is to be able to create their own distributions with minimal effort. Or, if a users wishes, they can simply install the base Scientific Linux release.