Caml is a strongly-typed functional programming language from the ML family. OCaml (Objective Caml) and Caml Light are two open source implementations of Caml developed at INRIA Rocquencourt, projet Cristal.
ObjectiveCaml (Ocaml for short) is a multiparadigm general purpose programming language : it is primarily an object oriented functional language, but it also allows to program in imperative style when needed. It is one of the fastest and most powerful programming languages around. It manages to be much simpler and much more safe than C++, while being about as powerful. It is also one of the best languages to learn WikiPedia:Functional_programming, and a lot of fun to hack.
Even if you don't plan to hack MLdonkey, give it a try !
http://caml.inria.fr/ - The Caml language
http://caml.inria.fr/pub/docs/manual-ocaml/manual003.html - for a quick glimpse to the OCaml syntax
There are tools and tutorials in DevelopersTips.