An attack upon an opponent in order to discredit their arguement or opinion. Ad hominems are used by immature and/or unintelligent people because they are unable to counter their opponent using logic and intelligence.
Person A: I think we should spend more money on environmental protection.
Person B: You just think that because you’re a stupid tree-hugger.
Person A: It is crucial that we facilitate adequate means to prevent degradation that would jeopardize the project.
Person B: You think that just because you use big words makes you sound smart? Shut up you loser; you don't know what you're talking about.