Dramatic irony is a relationship of contrast between a character's limited understanding of his or her situation in some particular moment of the unfolding action and what the audience, at the same instant, understands the character's situation actually to be.
Leaps and bounds separate that which is ironic and that which many people simply say is ironic. Christopher Warner wants to set the record straight: Something is ironic if and only if it is the exact opposite of what you would expect. See his lesson here.
Here's a comic book represenation of the three most common uses of irony (from The Oatmeal).
Irony is a rhetorical device, literary technique, or situation in which there is an incongruity between the literal and the implied meaning. No written method for indicating irony exists, though an irony punctuation mark has been proposed. In the 1580s, Henry Denham introduced a rhetorical question mark or percontation point which looks like a reversed question mark. This mark was also proposed by the French poet Marcel Bernhardt at the end of the 19th century to indicate irony or sarcasm.
Jaws is a 1975 American horror/thriller film directed by Steven Spielberg and based on Peter Benchley's novel of the same name. The prototypical summer blockbuster, its release is regarded as a watershed moment in motion picture history. In the story, a giant man-eating great white shark attacks beachgoers on Amity Island, a fictional summer resort town, prompting the local police chief to hunt it with the help of a marine biologist and a professional shark hunter.