The skam engine is written in prolog. Skam protocol files are compiled (using a perl script) to a prolog .P file. Skam protocol syntax is really just syntactic sugar for the underlying prolog program.
For an introduction to prolog, see http://www.coli.uni-sb.de/~kris/prolog-course/html/index.html or http://kti.ms.mff.cuni.cz/~bartak/prolog/contents.html