Verbal Irony - A figure of speech in which one meaning is stated and a different, usually antithetical meaning is intended.
In Tartuffe verbal irony is directed at or through Orgon. An example would be when Tartuffe is confessing his love to Elmire, and Orgon is secretly listening. After Tartuffe leaves and Orgon comes out of hiding, Elmire proclaims, "What, coming out so soon? How premature!" She is obviously using verbal irony to convey to Orgon that she wanted him to come out sooner.
Dramatic Irony - A discrepency not between what the speaker says and what he means, but between what the speaker says and what the author means. Dramatic irony can also be expressed as when the reader knows more than the actor.
The same example used above is an example of dramatic irony. The reader or watcher of Tartuffe knows that someone is under the table...which is something all of the actors do not know.
Irony of Situation - A discrepancy between appearance and reality, between expectation and fulfillment, or between what is and what would seem to be appropriate.
Irony of situation is used to end Tartuffe. At the end of the play, it seems like there is no way for Orgon to avoid going to jail because of Tartuffe, but it ends up with Tartuffe going instead.