What was the difference between being an Earl vs a Duke or a Count etc?
Depends on the time and place, of course.
In the Frankish empires of the early Middle Ages, a count was an administrator appointed by the king. Later, the office became territorial (i.e. each count had a county or shire to administer) and hereditary. Earl is simply the English word for count, they are considered equivalent titles.
A duke was in the early Middle Ages a military leader, i.e. someone whom the king charged with an army to fight a certain war. In the High Middle Ages, duchies became territorial and hereditary as well, so that the Holy Roman Empire was divided in about ten duchies. Dukes were considered to be higher-ranked than counts. In the feudal system, this could be seen by the fact that many counts (but not all) owed fealty to a duke, but there were no dukes who owed fealty to a count.
Eventually, the difference in rank became the only difference between dukes and counts. Additional ranks were inserted as well, e.g. a margrave (or marquis) was in-between a duke and a count. In the later Holy Roman Empire, there was a distinction between plain counts and prince counts -- princely counts being the ones who were directly subject to the king, rather than to a duke. In the Imperial Diet, dukes had a single vote each, whereas several prince counts had to share a vote, showing their lower rank. Non-princely counts were not represented in the Diet at all.
Ranks that are lower than count are baron and knight.
hi! as /u/antonulrich mentions, the answer(s) are going to depend on both the time and the place, so it would be helpful to potential respondents if you could specify the country/ies you're interested, and ideally some time bracket. Meanwhile, here are a few other related posts ~