Linguistic differences

[Curio Agelastus]

Salvete omnes,

Something I've been struggling to work out is the difference between a Cognomen and an Agnomen. As I understand it, both are additions to a person's first name and family name (Praenomen and nomen). However, Cognomina often seem to be inherited as well. So is a cognomen an inherited name that describes a characteristic, and an agnomen an awarded name? (Like... Britannicus!!! )

Or have I entirely misunderstood this?

Bene valete,
Marcus Scribonius Curio Britannicus.

[Quintus Servilius Priscus]

A good example of what you are looking at is Quintus Servilius Fidenas.
From what I have been able to find out about the real Quintus Servilius is he was one of the two Consular Tribunes that defeated the Etruscans at the city Fidenae. Hence his last name Fidenas for the City's conquest. Another
Servilius had the name of Gaius Servilius Ahala. Ahala supposedly was of
Etruscan origin and meant "Armpit". He killed a person who was thought to
have been a threat to Rome by pulling a knife from under his armpit area
and stabbed him(Brutus's mother was supposed to have been a Servilia
and that history of Ahala might have influenced him in Caesars assassination). He got the last name for his deed.
Quintus Servilius was Consular Tribune 6 times and Dictator once.

A Consular Tribune was a Military Tribune with Consular power.