Definition of irony:

Irony is a figure of speech in which words are used in such a way that their intended meaning is different from the actual meaning of the words. It may also be a situation that may end up in quite a different way than what is generally anticipated. In simple words, it is a difference between the appearance and the reality. Irony relies on the difference between expectation and outcome.

Verbal irony:

Verbal irony is the use of language to express the opposite sentiment than what is expected. The most recognizable form of verbal irony is sarcasm, where the speaker says the opposite of what they mean, often for comedic effect.  Irony which a person says or writes one thing and means another, or uses words to convey a meaning that is the opposite of the literal meaning.

situational irony:

Situational irony is when the exact opposite of what you expect to happen happens. Situational irony, like verbal irony, is powered by the incongruity between the expectation and the actual outcome. Irony involving a situation in which actions have an effect that is opposite from what was intended, so that the outcome is contrary to what was expected.

Dramatic irony:

Dramatic irony is when a playwright or a novelist creates an ironic situation that only the viewer or reader knows about. It is when the readers know something that the characters themselves haven't realized yet.  Dramatic irony is a literary/dramatic effect achieved by having the audience know more about the situation, circumstances or future of a character than the character himself.