R terminology for what are called library sections in S and Splus is packages.  There are two types of packages in R: core and contributed.

Because R loads everything into a workspace some of the less used functions in S have been moved to packages.  Here is a list with descriptions of the core packages: ctest, eda, lqs, modreg, mva, nls, splines, stepfun, ts

In addition to these core R libraries, there are many extra contributed packages.

List of builtin functions in Splus are available in the core and contributed packages.