In the case of ruling families, "Fürst" signifies the ruler, the head of the family. This applies to the Reuss lines, the Schwarzburg lines, Waldeck-Pyrmont, Lippe, Schaumburg-Lippe, the Hohenzollern lines, etc.
In the case of what are called "mediatized" families, the head of the family, and sometimes the head of individual lines, is often "Fürst", though sometimes merely "Graf" [Count].
In the case of some non-reigning and non-mediatized families, the head is a "Fürst". Examples are the Fürst von Pless or the Fürst von Carolath-Beuthen.
There are some anomalous cases of families in which all members are called "Fürst", such as the Princes von Wrede.
In all these cases "Fürst" is normally translated to English as "Prince".
To add to the confusion, in German, all members of many Russian and sometimes even Italian princely families are called "Fürst". And the Russian Grand Dukes, as we call them in English, are usually called "Großfürst" in German.