Levels of power:
1st | Imperator (Emperor) - ruler of a large country that devoured many kingdoms
2nd | King/Queen - ruler of a kingdom, can be large or not.
3rd | Duca - ruler of a large region of an Empire or kingdom. Former royalty can be duca.
A ducal is the territory ruled by a duca or the lands (and/or incomes) specifically attached to the ducal title. A duca is the title itself. These were initially established as essentially corporations holding properties that provide income for the ruling Imperator/King.
4th | Prince/Princess - heirs of the first 2 categories, sometimes 3rd, too. No actual power, only potential, unless the country in question is a Princedom, in which case equates to King/Duke.
5th | Marquess/Marquise - owner and governor of border regions. Have roughly equal official power as a Count, but more autonomy due to distance from capital and larger military (to defend from other countries).
6th | Earl or Count/Countess - owner and governor of a piece of land. A count is the principal figure of the region
6.1 Count-Palatine.
"Palatine" refers to extraordinary powers granted to a noble. The word "palatine" means a region under the authority of a noble where the rulers (usually the king's) writ was suspended. While the noble owed allegience to the ruler, the holder of a palatine had absolute authority, including the right to grant titles of nobility, create knights and raise armies -- i.e., powers normally reserved to a sovereign.
7th | Viscount/Viscountess - mostly honorary title, sometimes owning the land, but wielding less actual power. It's like Count-wannabe, literally vice-count.
8th | Baron/Baroness - owner or governer of a small region. Not terribly different from Count, but might have less land or limitations on hereditary transfer of land ownership. Sometimes not even owner of the land, but only the ruler/governor.
9th | Baronet - a) a knight who has hereditary rights (his children would get the title automatically); b) an heir to a Baron/Baroness. Usually doesn't own/govern the land.
10th | Knight - non-hereditary title for a person recognized as nobility by the King/Duke.
Further Details:
Think of a Baron as having 1 castle - or just a well-fortified stone manor - and 2-5 barons being vassals of a count.
A count almost certainly has a stereotypical castle, with 2-5 as the vassals of a duca. Some counts don't have a duca above them, and pay homage directly to a King/Imperator.
A Duca might have more than one castle and there may be 2-5 ducals in a kingdom. Kings usually have at least 2 or 3 castles, and there would need to be at least 2 kingdoms for there to be an Imperator.