Irony is a literary device that pits appearance or understanding against reality. Irony is commonly found in literature, poetry and plays, though it can appear in any form of writing or entertainment. Irony is often employed to point out a central idea or to cause the reader or audience to think more deeply about what is occurring in a story.
Verbal irony is the most common kind of irony in literature and in everyday life. This kind of irony is used when a person or character says the opposite of what they mean. Verbal irony can often be sarcastic, but not all sarcasm is ironic. In most cases, the contradictory nature of an ironic statement is meant to be very clearly understood. For example, a waitress drops an entire tray of food into the lap of a patron and her boss tells her, "Well done!"
Another form of irony is situational irony. This kind of irony occurs when the outcome of events is directly contradictory to what any reasonable person would expect to happen. O. Henry's short story "The Gift of the Magi" is a classic example of situational irony. In this story, a poor woman cuts her hair off and sells it to buy her husband a chain for his pocket-watch. Unbeknownst to her, her husband has sold his watch to buy her hair combs as a gift. Thus, their expectation of surprising each other with the perfect gift is subverted.
Irony is said to be dramatic when the audience is aware of a crucial piece of information that the character in a story or play lacks. A famous example of dramatic irony comes from William Shakespeare's "Romeo and Juliet." At the end of the play, in the crypt, the audience knows that Juliet is not dead -- she has only taken a sleeping potion. Romeo, however, walks in and finds her lying still on the floor. He believes that she is dead and, in his grief, kills himself just moments before she awakens.
Less tied to works of literature than verbal, situational and dramatic irony are, historical irony occurs with the benefit of hindsight. Historical irony allows people to look back at events in the past and analyze the contrasts between the anticipated outcomes of events and the actual outcomes. An example of historical irony can be found in the fact that the U.S. Supreme Court used the Bill of Rights -- adopted in 1779 -- to deny slaves their rights in the 1856 Dred Scott case.