Irony

The Definition of Irony is surprisingly non-trivial as it is enjoyable:

Here's About.com take on it: http://grammar.about.com/od/il/g/ironyterm.htm

The use of words to convey the opposite of their literal meaning; a statement or situation where the meaning is contradicted by the appearance or presentation of the idea. Three kinds of irony are commonly recognized:

• Verbal irony is a trope in which the intended meaning of a statement differs from the meaning that the words appear to express
• Situational irony involves an incongruity between what is expected or intended and what actually occurs
• Dramatic irony is an effect produced by a narrative in which the audience knows more about present or future circumstances than a character in the story