The various distinctions and differences between a duke, lord, sir, earl, baron, baronet, and prince were drawn long ago, but still hold true today.
Here they are in order from top to bottom:
Prince
Duke
Marquess
Earl
Viscount
Baron is the lowest rank of the upper classes, having earned this title by faithful duty to the king or feudal lord, usually in the form of military service.
A baronet is the highest-ranking class that can be bestowed upon a mere commoner like you or me. This title is also usually earned through faithful service to rulers.
A lord, generally, is any of the above who get a title and property by hereditary right or by being born into the right family. “Sir” is simply the title used when addressing a knight or a baronet.
Often, some hereditary titleholders will hold other titles as well.
For instance, the eldest son of the king or queen of England, aside from being a prince, is also automatically the Duke of Cornwall, no questions asked.
