The Roman name usually had three elements (tria nomina): a praenomen, a nomen or nomen gentilicium and a cognomen.

Note that in their use the praenomen correspond to our first name and the nomen to our surname, i.e. the only part of the name that is always inherited and that indicates our belonging to a family (note the French "nom de famille" for surname). However, as the word "surname" suggests, etymologically most family names stem from our equivalent of a cognomen, which could be a nickname (Bullock, meaning "young bull"), a place name (Ashley), a profession (Smith) or a honorific title, such as a title of nobility.

If we take the following example: Gaius Julius Caesar, then:

Gaius would be the praenomen or personal name, but of a lesser importance than our first name.  The same pronomina were used over and over again from generation to generation and by 100 BC only about 20 common praenomina were still in use among patrician families. The two most common Roman praenomina Marcus and Gaius are derived from the god Mars, the legendary father of the founders of Rome Romulus and Remus, and Gaea, the Earth Goddess. Tiberius comes from a sort of a deity, the Father Tiber.

Other common praenomina are derived from circumstances of birth: Lucius (the light of day), Manius (morning), Posthumus (born after his father`s death), Gnaeus („born", „birthmark"), Spurius (possibly used for bastards), Agrippa („born feet first").

The so called "number names" possibly refer to the position in the sibship: Quintus, Sextus, Septimus, Octavius, Nonus, Decimus (fifth, sixth, seventh, eigth, nineth and tenth son respectively). However, as they start with the number five scholars like the finnish philologist Olli Salomies questions the fact that „number names" were used to number sons in the family and utters the possibility that designated the month of the child's birth instead.

Some praenomina were exclusively used within the same family, for example Appius was used only by the Claudii and Tiberius only by the Claudii Nerones and the Aemilii.

Julius would be the nomen or gentilicial name, which designated the person's gens or clan. The original gentes were descended from the families that settled Rome. Well-known nomina include many of the familiar names of ancient Rome, such as Aemilius, Claudius, Cornelius, Julius, Pompeius, Antonius and Valerius. The nomen often ended in -ius, -eus or -us and were derived from a praenomen or a cognomen. Some nomina were derived from a place name or from other tribes, such as the Etruscans. 
Nomen in the following endings, for example, indicated a non-Latin origin: Gallic origin (-acus), Etruscan (-na, -nius), Oscan (-idius), Umbrian or Picene oigin (-ienus). The nomen was the most important element in the Roman naming system: it was the component of name that was inherited and also indicated the position of the gens in the state, its antiquity and sometimes ist origin. Indeed, the majority of nomina gentilicia come from names of tribes. Nomina could also have other references, for example, Barbatus (bearded), Crassus (fat), Calvus (bald), referred to physical characteristics; Maximus (best), Brutus (brutish) to personality traits or qualities; Mallius (ironworker), Faber (carpenter), defined occupations and Lupus (wolf), Corvinus (raven), and Cicero (vetch) had a symbolic reference.

The Caesar of our example would be the cognomen or surname, which could be honorific or designate a branch within the gens or just be a nickname, in which case it was also known as an agnomen. The cognomen was not used officially until around the time of Sulla. Cognomina were sometimes inheredited for the sake of convenience or to keep a honorable name within the family.

Ocasionally a fourth name was added to the tria nomina, for example in case of adoption all three names of the adoptive father were transmitted to the adoptee, and an adjectival form of the adoptee's own nomen (formed by adding the suffix -anus) was usually added. Thus, when Gaius Octavius Thurinus was adopted by his great-uncle, Julius Caesar, his formal name became Gaius Julius Caesar Octavianus. Note that this fourth name was sometimes used in a derogative or disrespectful way.

Modern historians usually refer to him as Octavian when referring to the period before he officially gained the honorific Augustus ("the revered one") which was added to his name in 27 BCE. This title was also referred to as  cognomina ex virtute and was usually a title of honor given to a great politician or a victorious general. In the latter case, the cognomina ex virtute often was a declination of the region conquered. P. Cornelius Scipio, for example, was subsequently known as P. Cornelius Scipio Africanus after he had defeated Hannibal.

Women were named with the feminine form of the nomen (or, occasionally, the cognomen) that indicated her gens. In public they could be identified by the genetive form of their father's cognomen. Sometimes a term in the form of an adjective (Maxima, Secunda) or diminutive (Livilla) was added to distinguish between sisters. Sometimes the mother's or husband's name was added to the full name of a woman.

External links:
- Roman Nomenclature by Barbara F. McManus, College of New Rochelle
- The Roman Name by John Porter, University of Saskatchewan
- Roman Names
- Yonge Charlotte M., The History of Christian Names,1863
- Hanifin, Michael J. F. and Leonard L.N. Ashley. Onomasticon of Roman Anthroponyms: Explication and Application. Names. 1978; 26:297-341.
- Roman Name Convention in the Late Roman Republic