What is the difference between authoritarian and totalitarian?

An authoritarian regime is a system where power is concentrated in the hands of a non-elected or pseudo-elected elite/bureaucracy acting discretionary and with no or little regards for rule of law or individual rights of any kind. In an authoritarian regime, ideology is not a main driving force of society (even if it is present), and while freedom is severely curtailed, some limited arenas of expression typically survive. Examples of authoritarian regimes: Tunisia, Egypt, Russia, Kazakhstan, Burma...

A totalitarian regime is a system where power and society are subsumed to an ideology. The state tries to control and regulate all the actions (and thoughts) of its citizens in accordance to that ideology, eliminating anybody who is against it. The power of the state over its citizens becomes total, as does the power of ideology. Examples of totalitarian regimes: USSR (between the 1920s and the early 1980s, but especially in the 1930s and 1940s, with communism as their main ideology), Nazi Germany (following fascism and national-socialism), Italy in the 1920s and 1930s (following fascism), North Korea (with juche communism as their main ideology).