Irony is the expectation of one event and another, a completely different event that happens and still makes sense. There are three types of irony: verbal, situational, and dramatic. Verbal irony is when someone says one thing but means the opposite and everyone understands that that person means the opposite of what he/she are saying. Situational irony is the opposite of what is expected to happen, happens but it still makes sinse. Dramatic irony is an audience who knows something that someone else doesn't know.
The importance of irony in modern art and literature and, more latterly in the intelectural sciences and in culture generally, can hardly be overestimated. For some writers the cultivation of irony is the most essential qualification for any thought, any art or literature or social or political therory to be truly modern.