The use of words to mean something different from what is actually said.

Example 1: "This car seat is as comfortable as a pile of rocks."

Example 2: In Jonathan Swift's "A Modest Proposal" he suggests that people should eat babies as a solution to famine.

When what happens is opposite to what is expected to happen.

Example 1: A St. Bernard dog is trapped in an avalanche.

Example 2: A marriage counselor gets a divorce.

Example 3: Adrift in the middle of the ocean without any potable water.

When the full significance of a situation in a story is understood by the audience but not the characters themselves.

Example 1: In Romeo and Juliette, the audience knows Juliette isn't dead when Romeo poisons himself.

Example 2: In a horror movie when a character goes into a house where the audience knows a murderer is waiting.