A FormalLanguage is a set of symbols and a set of rules for combining those symbols to form valid words and/or sentences.
By associating the symbols (and possibly words and sentences) in a FormalLanguage with elements of a given system -- i.e., by assigning them an interpretation -- we can use a FormalLanguage to unambiguously express definitions or relationships in the system.
Contrary to some belief, a FormalLanguage is not the same as a WoodenLanguage or a WoolenLanguage in human language!
See http://en.wikipedia.org/wiki/Formal_language and FormalSystem