Irony is when the something counter to the expected happens.
We look at three types of irony: Dramatic Irony, Situational Irony, and Verbal Irony.
The example above is verbal irony as the reader assumes Nemo is the fish's name.
The example to the right, with the cat and the fish, is Dramatic Irony because the audience (us) can read, so we know that the cat is seasoning his future food, not being nice.
The above is an example of situation irony as we expected the Coyote to be flipped by the weight of the rock.