Roman à clef

A roman à clef or roman à clé (French for "novel with a key") is a novel describing real-life events behind a façade of fiction. The "key", not present in the text, is the correlation between events and characters in the novel and events and characters in real life.

The reasons an author might choose the roman à clef format include:

Satire;
Writing about controversial topics and/or reporting inside information on scandals without giving rise to charges of libel.
A roman à clef also gives the author the opportunity to turn the tale the way the author would like it to have gone.
Since its original use in the context of writings, the roman à clef technique is also used in the theatre and in movies, like The Great Dictator depicting Hitler and nazism.