There were definitely treaties and peace agreements. In the specific case of the Hundred Years War there are several breaks in the war that were the result of treaties negotiated for a fixed number of years. It was more common to agree to not wage war for an amount of time (say 10 years) than to agree to a complete cessation of hostilities. It wouldn't really have been realistic to expect war to stop in the Middle Ages, people fought a lot.

There weren't really Ambassadors or Diplomats as proper jobs. Kings would send nobles from their court to act as an ambassador to another court but it generally wasn't a job someone would do for their whole life. The church was commonly used in negotiating peace treaties since it was in the church's interest to stop warfare between Christians. Bishops and Archbishops were the best suited to this but in extreme cases the Pope might get involved.

Froissart provides some description of a bishop attempting to stop the impending battle between the French and English in his description of the Battle of Poitiers. Froissart in general is a good read if you want an idea of medieval life/politics/war during the first part of the Hundred Year's War.

As a broad answer to your question diplomacy was pretty sophisticated, at least between monarchs. The life of Edward I is an interesting example since he had extensive dealings with multiple kings of Scotland and France and seeing the differences in their relationships is quite interesting. Michael Prestwich has written extensively on Edward I as well as his successors and Marc Morris' biography of Edward is a very readable book that spends a lot of time on Edward's wars and the negotiations around them.