One of the most misused words in the entire English language.

There are several types of irony.

Socratic irony - When someone pretends to be naive about a certain subject, and uses his questions about it to point out a flaw in the established belief. This is often used on the TV show South Park, where the children often ask questions about a situation until the folly in a parent's decision becomes clear.

Sarcasm - Understatement, mocking overstatement, or heavy-handed irony (stating the flat opposite of the truth) where both parties are aware of the difference between what's said and what's actually happening.

Situational Irony - The irony that most people think of. A difference between what you expect to happen (in a story, for example) and what actually happens. Rain on your wedding day would be a sort-of example, because a wedding day is generally expected to be a perfect, happy day. The good advice you didn't take, however, would NOT be irony, because that has nothing to do with what is expected and what isn't expected. A traffic jam when you're already late wouldn't be irony either; there's no automatic expectation that traffic will be fine, just because you happen to be late.

Irony of Fate - The concept that the Gods, Fates, etc. are toying with humans for amusement by using irony. Beethoven's loss of hearing is a famous example; one would expect a composer to be able to hear his compositions, but fate denied him that ability.

Tragic (Dramatic) Irony - When the audience knows something that some of the characters don't know in a play/movie/novel/whatever. For example, when the horror flick psycho is in the house and the homeowner just goes in without suspecting anything.

of, resembling, relating to, or tasting much like iron

the opposite of what is intended happening or being presented.

America "Bombing for Peace"

NOTHING in the Alanis Morissette song "Ironic".

Wearing a Che Guevara print t-shirt or similar merchandise produced by a multinational corperation.

A statement or situation where the meaning is directly contradicted by the appearance/presentation of the idea.