“Authoritarian” is best understood by considering where authority/the power rests. Is it with the government/state? Or does it sit with the individual? The opposite of “authoritarianism” is “liberty”. You may have countless forms of government that support one side or the other, or anywhere along the spectrum between the two.
That said, totalitarianism is an extreme form of authoritarianism, implying total control over all affairs. "Despotic" is the degree to which those in control are willing to go to enforce their authoritarianism, often implying terrorizing the populace, even arbitrarily, as a means of gaining compliance/achieving those in power's desired ends (which may be as base as pure, morbid self-gratification, etc.).
That aside, a dictatorship (generally synonymous with autocracy) is a form of government — as are democracy, monarchy, anarchy (classic definition, not the "hoodlums busting windows chanting anti-capitalist/pro-commie slogans" pop-media usage), theocracy, etc.
Any one of these forms of government can tilt towards liberty or authoritarianism.
For example, a dictator might be a despot and deem that the nation he rules is for his personal agenda only, that people are there to serve his interest (financing it through taxes, for example), the populace essentially slaves to his whim, his national interests, etc. He and his cronies may rape and pillage and terrorize, their own populations etc. as a means of gaining compliance or/and personal gratification.
Or… the dictator may deem that liberty is the highest social order, and that the sole purpose of his government is to restore the population's liberty — their right to consent. He would jail those who would steal, murder, etc., and while he would encourage the to be charitable and good people, and otherwise leave them alone. The people would be responsible for funding the forces used to protect their own liberty, and would be free to organize that as they choose. He would probably come down on hard against those who would choose to politically organize to create a form of government that enables the violation of the individual's right to consent/liberty. In other words, the people are able to do as they please so long as they respect others' liberty — their right to say "no thanks" when it comes to their property, body, labor, etc. They may have no say in their government, but their government only intervenes to protect them. It is otherwise non existent. This is unlikely, but it is conceivable.
With that in mind, totalitarian quantifies the degree of which authoritarianism reigns. E.g. a democracy may lean quite heavily to authoritarian rule — where the majority may violate the liberty of others who lose elections. The more the people vote that individuals must defer to the State authority on all matters, the more totalitarian the authoritarian government.