The document discusses context-free grammars (CFG) and their efficient implementation using difference lists, a Prolog technique. It explains how difference lists represent grammatical categories as the difference between two lists, allowing a sentence to be recognized by consuming the symbols in one list and leaving the symbols in the other list. The document then presents a CFG recognizer implemented using this difference list approach. It also introduces Definite Clause Grammars (DCGs) as an alternative notation for writing grammars as Prolog programs.