What are the central features of absolutism? In what ways does the absolutist state differ from the medieval state?
In theory, an absolutist state is one in which the monarch has complete, total authority over society, and can effectively do whatever they want without anyone to stop them.
In practice, no real-world state has ever been that absolutist. Even extremely powerful kings have been restrained by economic, social, and political limits. Often even very powerful kings fear that the aristocracy will undermine them, or that the general populace will try to overthrow them, and as a result they make some effort to please their subjects rather than simply fulfill their own aims.
There is actually some dispute among historians over where exactly to draw the line as to which states were "absolutist" and which were not. Many monarchs asserted absolute authority in principle but were never able to obtain in it practice. Still, some governments have gotten much closer to absolutism than others.
The chief difference between an absolutist state like France in the mid-18th-century (or Saudi Arabia today) and the medieval state is their level of centralized power. During medieval times, the feudal system reigned, and power was much more decentralized. Travel and communication between regions were so costly that it was difficult for national authorities such as kings to really exert power over their subjects. Instead they relied on local authorities such as dukes, and to greater or lesser extent sought to influence those local authorities rather than control them by force. This gave kings much less direct power over individual subjects, though as far as the subjects were concerned it may not have made much difference in their lives.