CERN Scientific Linux is based on Red Hat Linux, with modifications to the kernel and with additional software for High Energy Physics (HEP). Scientific Linux includes software suited for scientists and people working with scientific data. This includes FITS libraries, Graphviz, and R.
SL is a Linux release 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.