Communist dictatorships are different in that they try to create a communist state. They are trying to create a state in which there is no private property and in which the government controls economic activities of all sorts. Not all authoritarian governments are trying to achieve these ends.
All communist governments that the world has seen have been authoritarian. However, not all authoritarian governments are communist. You can have, for example, military authoritarian governments such as existed in Egypt until recently. This government did not imppose communism, but it did try to control the political sphere so that there could be no dissent against it. You can also call governments like that of Iran authoritarian in that they try to impose certain religious behaviors on all and they also do not allow dissent. Yet, at the same time, they are not communist in any real way.
Authoritarian governments try to impose their will on their people and force them to behave in certain ways. Communist governments do this, but in a particularly communist way that involves efforts to create a centrally planned economy run by the government.