An emperor is a ruler or a monarchy that rules over an empire. An empire is a collection of people and nations ruled over by one monarch – the emperor. For instance: The Holy Roman Empire Stretched from the Lowlands and Germany to Italy, pieces of Slovenia, Austria, Poland and Czechia.
The Empress is a title given to the wife of the emperor, the mother, or women who rule in their own right.
An empire can have an emperor but also multiple kings.
A king is the male ruler of a country. His female counterpart is called a queen. These kingdoms could be sovereign (like France) or part of a bigger empire.
The titles of Archduke and Archduchess are only found in the Holy Roman Empire as it is a title that was given to the rulers of the Archduchy of Austria. It means: The most important Duke.
Grand Duke is a title that resides under Emperor and King but above sovereign Prince and even Grand Prince. In Europe we can still find a Grand Duke ruling over Luxembourg and historically Grand Dukes have ruled over Tuscany, Baden, Oldenburg, Saxe-Weimar, etc
A duke rules over a duchy. This title was widely known and used in Europe. In Germany, the title was better known as Herzog and in the Lowlands (Belgium and The Netherlands), people would say Hertog. The title has survived until the day of today and in the United Kingdom, members of the Royal Family would often be granted a Ducal title.
Also known in Europe as Markies, Margrave, Landgrave, Landgraaf and Count Palatine. Be aware: A woman holding the title of a marquess is also called Marquess while the wife of a marquess is called a marchioness. The Marquess is a nobleman holding a march (a county) on the border of the country while the count would have land further inward. A Marquess is more important since the ruler would have to be able to trust a Marquess to guard the borders and thus a Marquess stood very close to the royal family.
A count, also known as an earl, holds a county in the centre of the country. In the United Kingdom, for instance, counties still play a very big part as they are now seen as some sort of provinces : for instance the county of Hampshire or the County of York.
A viscount is a rank used for a noble of lower to middle rank. In many countries, the title of viscount is non-hereditary, this means that the son of a viscount will not be a viscount. The title is granted by the king.
Baron comes from the french Baron and the latin Baro which means soldier or mercenary. In the early middle ages and the roman empire, these titles were granted to soldiers that did particularly well, the equivalent of a current day medal. Obviously, now the title is just a hereditary ( and non-hereditary!) title for lower nobility.
Knight is a non-hereditary title for people that served the country well OR a title granted to soldiers of noble blood that are not in line to take the title of their dad. Obviously, even if the title was granted to you by the king, you would have to be quite rich since you would have to afford a horse, a page and expensive armour.