Obviously these are "ideal types" - not every regime fits neatly in a single category. Fidel's Cuba falls into ambiguous territory beyond traditional dictatorship but short of the mass violence associated with Totalitarianism. As presidential power expands beyond the reach of congressional oversight under Bush the Younger, the US gets further away from constitutional liberalism as traditionally understood - but no sober analysts would call the US an autocracy.
Constitutional Liberalism: A form of government that protects individuals' political liberties by dividing state power between genuinely independent institutions empowered to check and balance one another. Though it's often used as a synonym for "liberal democracy" there's no necessary link between the two: Britain practiced Constitutional Liberalism for hundreds of years before instituting democracy. In constitutional liberalism, state institutions display strong functional independence, and act under the authority of law.
Examples: Chile, Norway, Japan, South Africa, Canada (just to pick one for each continent.)
Autocracy: A form of government where all state power is concentrated in a single person. Formally independent state institutions are, in reality, dependent on the autocrat. Authority stems from the leader, not from the laws. An autocrat centralizes state power, but political entities outside the state retain capacity for independent action. Individual political liberties are curtailed but not entirely eliminated.
Examples: Venezuela under Chavez, Argentina under Peron, Russia under Putin, Egypt under Nasser, Singapore under Lee Kwan Yew.
Authoritarianism: Roughly synonymous with dictatorship, a form of government where all political activity is dominated by a single actor, through violence when necessary. The basic difference between autocracy and authoritarianism is that an authoritarian regime extends control not just over the state, but over all political activities in society. Political opposition is systematically repressed through the application of state power.
Examples: most traditional dictatorships in Latin America (Gomez, Trujillo, Porfirio Diaz, Batista and a very long etc.), also Iran under the Shah, Zaire under Mbutu, Indonesia under Suharto.
Totalitarianism: A form of government where all social life is dominated by a single actor through the systematic use of violence. Though often misused as a rough synonym of "authoritarianism," totalitarianism is something fundamentally different. Totalitarian regimes seek to control all aspects of social life, not just politics. Whereas authoritarianism is "rational" - in the sense that it does what is necessary to maintain political power and no more - totalitarianism is not.
Examples are few: France under the reign of terror, Hitler, Stalin, Pol Pot, and North Korea under the Kims.
Chavez, on the other hand, is not a borderline case at all. He's an autocrat through-and-through. Though he's made some worrying moves in the direction of authoritarianism, it's perfectly clear he's far from controlling all political life in the country. It's equally clear that he's centralized all state power under his authority.