Scientific Linux ("SL") is a Linux release put together by Fermilab, CERN, and various other labs and universities around the world.
The base SL distribution is similar to Red Hat Enterprise Linux, recompiled from source. Various labs will customize SL, and provide their own customized version called a "Site" which is primarily used that Lab (e.g. the Fermi "Site" is used primarily at Fermilab in the US.)
The main goal for the base distribution is to have everything compatible with Enterprise, with only a few minor additions or changes. SL does not aim to be "100% binary compatable" like CentOS, but instead aims to be as close to the commercial enterprise distribution as we can get it.
See www.scientificlinux.org for more information.