Tu quoque is a criticism of one's accuser by pointing out the accuser is also guilty and therefore a hypocrite. It is used to divert criticism and change the subject of the debate.
A latin phrase meaning "Thou, also", this argument asserts or implies that a certain position is false or should be disregarded because its proponent fails to act consistently in accordance with that position. It is an attempt to show that a criticism applies equally to the person making it, and, as such, can be considered an ad hominem argument, since it focuses on the opposite party itself, rather than their position. It is also a form of red herring.