Titles... Lords, Ladies, Dukes etc

Dukes are on top of the totem pole, you'll notice that certain members of the Royal Family are also Dukes, Duchesses.
They are hereditary and have been around a long time, most of them.
Lords and Ladies can be a mixture of hereditary and the title bestowed by the monarch, these days at the recommendation of the government. They're pretty much ten a penny whereas a Dukedom isn't all that common.

I think that's about right...best thing to do is have a look at Debretts Peerage, it's all in there.

English Peerage ranks

My google-fu is strong

http://en.wikipedia.org/wiki/Peerage

Note, this is ENGLISH peerage, but other European countries are similar. Some differences notably in Spain and I think Norway.

This is my favorite run-down:

http://www.members.shaw.ca/jobev/title.html

French historical nobility is also not like English. They had counts and viscounts in the middle ages, whereas the English did not.

I believe the medieval hierarchy was: king, duke, earl, baron, knight. There are viscounts and marquesses later on, but not in the 1400s where my knowledge lies.

Here's a great link - one of the more in depth I've found so far for the British peerage.