Totalitarianism is full rule by the state, often by a single strongman. Such movements depend on almost total penetration of the cult of the leader and include domination of every aspect of society from the economy to the military to mass media.
It was Giovanni Amendola in the 1920s who apparently first used the term in describing Italian Fascism, which he claimed was fundamentally different than normal dictatorships and a great deal more pervasive in terms of how the political system was implemented and presented.
In the modern era, totalitarianism has come to have very negative connotations for Western ears. It conjures up the worst abuses of dictatorial states such as the USSR and Nazi Germany. Regimes such as that of Pol Pot in Cambodia are said to be totalitarian with overtones of inevitable genocide and bloody suppression of any alternative points of view.
Totalitarian states are seen as far more dangerous and uncompromising than "authoritarian" ones. The authoritarian state may have a dictatorial leader but the all-encompassing nature of the state itself may not be manifest. An authoritarian state is one that uses propaganda and other dictatorial mechanisms for purposes of empowerment but it is possible within the larger definition of "authoritarian" to envision at least some competing power centers. A totalitarian society has no competing power centers and brutally represses and stamps out any hint of dissent.
Authoritarian environments are often seen as involving a monopoly of government power but this is rather different from totalitarian societies that attempt to reorganize the larger society as well. Totalitarian ideologies leave little or no room for competing ideas. It is made clear that those living in such societies must adapt not only to the outward sociopolitical structure but must also internalize the larger ideological manifestations.
Authoritarian societies are often said to be less ideological, more pluralistic and predictable in terms of their exercise of power. Totalitarian governments seek to invade every part of an individual's life with the idea of providing a seamless mesh of propaganda and state-initiated activities that leave little or no room for thinking outside of proscribed norms. Authoritarian societies may be tolerable ones; totalitarian societies rarely are.