I didn't understand feudal ranks when I first started working with this material, and found it quite a hindrance, till I came across this phrase: Do Men Ever Visit Boston? It's quite difficult to read any historical material without some feel for feudal ranks and titles. Here's a very brief explanation:

• Duke: Think of as governor of a region, taken almost directly from the Roman (Latin) Dux. In practice, the strongest Duke was usually the King, many of the Dukes would be related to each other, and the Dukes would spend a lot of time fighting with each other over who would be king. England, due to its small size, had relatively few dukes.

• Marquis: Think of as a military governor. Governor of a newly conquered region, border regions, etc. In some cases, a title without the land (the title holder has to figure out how to conquer it!). Expected to be in a state of constant war. Many spellings. From the French for the military "marches"?

• Earl: A Count, in all of Europe except England. Think of as a governor of a county. The wife of an earl is a countess. The Normans apparently did not get the conquered anglo-saxons to adopt the term count (a sign of the continued cultural amalgamation that was to continue to occur). Anglo-saxon society, very roughly, contained earls, ruling landlords ( house carls) and serfs ( churls).

• Viscount: The Vice-Count. Pretty much what it sounds like; in charge of the county when the Earl was away at war or at the court of the King or Duke. We get our Vice in Vice-President similarly...

• Baron: Responsible for all the land in the county. Think of the county as being divided up into a relatively small number of large plantations, each run by a baron. A less-flattering but more formal version of the title is tenant-in-chief. In England, in theory, the King originally owned all the land and the barons were granted the right to "work" one of the plantations by the whim of the King. All the titles above that of Baron effectively form "the Administration". The Baron probably leases much of his land to his sub-tenants, and is responsible for providing a fixed number of knights to "the Administration" (it's as if he paid his taxes in combat soldiery). If capable, he would command his own men (that is, serve as an officer). By the end of the Hundred years war, many Barons in England effectively had their own small standing armies and were able to challenge the authority of the King. A county would typically contain from 5 to 20 baronies.
  
  

http://www.montaguemillennium.com/familyresearch/ranks.htm