Note that the RcppClassic package has been deprecated since 2010, all new development should use the Rcpp package instead.
Two APIs are supported. The first is an older API which was first introduced mostly in 2006 and extended in 2008. This interface is used by a few other packages and will be supported going forward in the RcppClassic-package package. A second and newer API that was started in 2009 offers more functionality, see the Rcpp-package package.
The RcppExamples package provides some simple examples for use of Rcpp.