In totalitarian states, the population are told what to do under threat of punishment.
In constitutional states, the government goes by a set of rules (the constitution) that guarantees the rights of the population.
I like this as the short answer:
In a totalitarian government, the people serve the government; in a constitutional government, the government serves the people.
At least that's the ideal. Some constitutional governments, both of the left and the right, seem to be forgetting that they serve the people, not the other way around.
Totalitarian states have one ruler and one party E.G. Nazi Germany. Russia was an Autocracy, the Czar ruled absolute the duma were no more than advisers.