Authoritarian is basically when someone has a desire to control people, often being used in a wider sense to describe a political regime. Of course a boss of a company can be authoritarian or a parent in the home can be too, it's just that the main connection people make is with the notorious regimes of the 20th century in Germany and the Soviet Union, of course they are just two examples. Authoritative is more positive and is basically a word to describe someone who has authority and knowledge built from experience. People can trust this kind of person. An authoritative source on authoritarian regimes would be, for example, a renowned historian. An authoritative scout leader would be someone who leads by example and shows positive patterns of behaviour for people to follow.