In the general sense, the German word "Fürst" refers to a ruler, as in Machiavelli's The Prince. Thus a king, a duke, and a Fürst in the narrow sense are all covered by the term. Before the 12th century also counts were included, according to how the word was used in Germany.
The word Fürst designates the head (the "first") of a ruling house, or the head of a branch of such a house. The "first" originates from ancient Germanic times, when the "first" was the leader in battle.
The child of a Fürst (in this general sense) is as a rule called Prinz (male) or Prinzessin (female), although exceptionally there exist families where all or some members are Fürst/Fürstin (Wrede) or Herzog/Herzogin (Anhalt, Bavaria, Mecklenburg, Oldenburg, Saxony and Württemberg).
The title Fürst is used for the head of a princely house of German origin. Unless he also holds a higher title, as duke or king, he will be known as Fürst von plus the geographic origin of the dynasty, alternatively Fürst zu plus the domain ruled. (Exceptions exist.)
The actual rank of the holder of a title is, however, dependent on not only the title as such, but on for instance the degree of sovereignty and on the rank of the lord of the title-holder. But also such matters as the age of the princely dynasty play a role (note the terms Uradel, Briefadel, altfürstliche, neufürstliche; and see German nobility).
Großfürst is German for the Latin Magnus Princeps, which in English is translated as Grand duke, used for instance for the sons of a Tsar. Grand duke is otherwise translated as Großherzog in German, and as Magnus Dux in Latin.
Example of bearers of the Fürst-title are the present-day rulers of the principalities Liechtenstein and of Monaco. Also the hereditary rulers of the former principalities of Bulgaria, Serbia, Montenegro and Albania bore the title "Fürst" until they gradually acquired the title of "King".
See also: Grand duchy, Grand duke, Grand duchess, Ranks of nobility and peerage and Titles of nobility