Roman Titles

Romans recognized three orders: patricians, equestrians and plebeians, and earlier, before the foundation of the republic, a fourth: royalty. Added to this, there was the concept of nobilis; to be noble meant you were descended from someone who had been Consul; being a patrician was necessary to become Consul (though you could buy your way in), but to be noble was ineffably grander, at least to the Roman way of thinking. This has been turned around a bit in Italy; in Italian cities today, a patriciate exists which is considered to be above mere nobility.
These notions of the Romans apply to present-day parlance. In the British system, one can discriminate between royalty, nobility, knights, gentry and commons: five grades. The Germans tend to regard certain of what the British regard as gentry as noble, and at the highest levels, what the British define as noble resembles what the Germans regard as "princely" and in general, continental systems as a whole tend to have a broader definition of noble.
In essence, the nobility were the landowners. To be a landowner you had to be prepared to defend your right to own that land, and with the progressive disorders that lead to the fall of the Western Roman Empire and the development of the feudal system in Europe, nobility became synonymous with the military caste an essentially self appointed caste.
In the West, it is nearly impossible to trace any noble lineage back much before AD 800 (though the old Gaelic nobility of Ireland has a special claim to antiquity here); anything before 1100 is remarkable. The organized system of titles we have today is a rather late development, but count and prince go back to the Roman Empire. Only when it was recognized that one might have betters (i.e., with the organization of nation-states) did the nobles start paying attention to titles, styles, and pedigrees.
A distinction needs to be made between nobility and peerage. In the British system, a peer is the holder of the title, while a noble is a member of a family headed by a peer. In the UK, such family members, while noble, are still technically common, which is not necessarily the case elsewhere. More narrowly, a peer also sits in parliament, as with the British House of Lords or the former French House of Peers. There are some titles in the United Kingdom (e.g., the Irish peerage, when the peer lacks another English, Scots or UK title) which do not permit one to sit in the House of Lords; thus, in Scotland, the distinction of a Lord of Parliament. 

There are two ways to earn a noble rank in most countries: birth or earning. Each noble rank has certain prerequisite statuses that someone must possess in order to achieve that title as well as having bonus statuses that you gain for having this rank. There are usually additional requirements for achieving nobility; it is never automatic simply for having the required status. If you are born into a title then the prerequisite statuses are added as additional bonus status. Some titles (such as knight) you cannot be born into and must earn. The exact structure of a country's nobility will vary from location to location, but the rough composite feudal order follows:

Pharaoh
Emperor/Empress
High King/High Queen
King/Queen
Kahn
Prince/Princess (royal)
Prince/Princess (city)
Archduke/Archduchess
Duke/Duchess
Marquis/Marquese
Chieftain
Count/Countess/Earl
Jarl
Subchieftain
Viscount/Viscountess
Baron/Baroness
Baronet
Knight
Hetman
Squire

Imperial Titles

Emperor comes from the Latin imperator, roughly commander, a title which ancient Roman armies spontaneously hailed a victorious general by; this entitled the general to a triumph (a sort of ancient Tournament of Roses Parade and Bowl Game). It was one of the titles of the Roman Emperor. Diocletian divided the Roman Empire into East and West, with two emperors, each emperor being Imperator and Augustus. Each co-emperor had associates, termed Caesars. This system proved unworkable, but the division of the Empire was permanent by about AD 395. The fall of the Western Empire is traditionally dated to August 23, 476 when Odoacer was crowned King of Italy. Romulus II, or alternately, Julius II Nepos, is considered the last Western Emperor. Caesar as a title of the Roman Emperor, or an associate of an emperor, entered both German and Russian as the word for emperor (respectively kaiser and tsar); the Bulgarian word tsar is translated to king. In English and the western Romance languages, imperator was the word that won out. The Byzantine/Eastern Roman Empire continued up to 1453, when Constantinople fell to the forces of the Ottoman Turks under Mehmet II. The last Byzantine Emperor was Constantine XI. It has been said that as Rome began and ended with a Romulus, so Constantinople began and ended with a Constantine. On Christmas Day, AD 800, Pope Leo III crowned Charlemagne Roman Emperor, i.e., Western Emperor. The Eastern Emperor (by now, the Byzantine Emperor) acceded to this. This is the start of the Holy Roman Empire, which would continue for a thousand years, until 1806. This was the first thousand year reich. 
Charlemagne is Old French for Charles-le-magne or Charles the Great. In German, he referred to as Karl der Große. In Latin, this is Carolus Magnus. He is counted as King Charles I of France and as Holy Roman Emperor Charles (or Karl) I.

Ruling Titles

The Holy Roman Empire and German system

The Holy Roman Emperor was an elective office. However, dynastic politics made it effectively hereditary, first with the Hohenstaufen, then, except for a brief period, the Habsburgs. For the Habsburgs, an imperial election was still necessary for an emperor to enter into his reign, and this was done by the Electors (in German, Kurfürst, Kurfürstin). The number of electors grew over time; at the dissolution of the empire these were:

The power of the Holy Roman Emperor was limited and the nobles who putatively owed him allegiance were oftentimes sovereign in all but name, particularly in the latter stages of the Empire.

Prince and Fürst

Prince has a long history. When the Emperor Augustus accepted the title from the Roman Senate, it meant first among equals, as in primus inter pares. It remained one of the titles of the Roman Emperor. The English word principal retains some of this meaning. In German, the idea has been translated into the title Fürst. In the most general terms, when not referring to the children of a king, prince refers to a sovereign or semi-sovereign individual who has direct personal rule over a relatively small territory. Because the Germans were much more used to minor princes than were other European states outside the Empire, and because German has the additional title of Fürst, Prinz in German does not have the very royal cachet it does in English, and sometimes may be classed as a lesser title than Herzog, or Großherzog depending on a particular title's history. Fürst is a uniquely German title that is best translated to prince and should be regarded as superior to prince. It designates the head of a princely house, or the head of a branch of such a house. Electors of the Holy Roman Empire were termed Kurfürst, Kurfürstin. Großfürst is the word used in German for a Russian grand duke (son of a tsar).

Dukes

The Latin dux was a military title that might roughly translate to field marshal. The stories of King Arthur probably refer to a dux bellorum in charge of the forces holding off the barbarian onslaught in early post Roman Britain. The English kings introduced the French ducal structure into the British system, and it was initially a mostly royal title (as all new creations during this century have been). In France especially after 1600, however, as well as in Britain, it has evolved into a mostly non-royal title. The idea that a duke is a royal title, however, is strong in Germany, perhaps stronger than it ever was in Britain, where all the children of the head of some ruling houses are automatically a Herzog or Herzogin, much as imperial offspring were archdukes or archduchesses. A duchy (or grand duchy) is the territory ruled by a duke (or grand duke) or the lands (and/or incomes) specifically attached to the ducal title. A dukedom is the title itself. Duke is normally a very exalted title. 

Marquess

This title glosses to march lord  i.e. a noble in charge of the marches (the border regions) of a realm in distinction to other lords in more-settled lands. These were essentially warlords with broad powers and in this context, may be thought of as a palatine title. In earlier times, it was a rare title; it was later revived as a grade between count and duke. As a senior title (about two-thirds of British dukes are also marquesses), it is not that common the United Kingdom, at least when compared to other countries. In Germany, margraves were ruling, heading there own little states and today are still accounted as formerly ruling houses.

Earl, Count, and Graf

Earl is related to Old Norse jarl, and is equivalent to count, which itself comes from the Latin comes. This in turn is related to the English word county, which pretty much explains what a count was: the principal figure of the county. In Roman times, the comes was a courtier, an Imperial official, and actually outranked a dux (duke). The German word Graf is etymologically related to the English reeve, which comes from the Old English gerefe. A reeve is an important appointed official, as with the shire reeve, i.e., the sheriff. What English divides among several words, German uses a single word with prefixes, and generally it has a broader meaning than English earl or count. 

Viscount

This title is mostly confined to the United Kingdom and France, though it appears rarely in Italy and elsewhere. This is the leftover title, what the king bestowed on someone who was not important enough to merit being made a count. It's a rather late innovation. It originated in France, as the count's deputy, i.e the vice-count.

Baron

Barons were originally (in Britain) those who held their lands directly from the king. Not all British nobles have baronies and many viscounts, for example, do not. The majority of the nobility in Britain are just plain barons. In the UK, life peers are always barons or baronesses. Once, a baron was an important noble, especially before the Renaissance. It was the barons who brought King John to heel at Runnymede, and robber-baron has entered English as the term for one of the lords who collected tolls from Rhine river traffic. In olden times, when there was little differentiation in degree or rank between neighboring nobles, baron could signify any noble, large or small, a meaning with some currency today on the continent, roughly equivalent in meaning to peer or lord in the UK. The terms Baron, Baronin, can be found in use in German lands, but apparently not that commonly. Freiherr and Freifrau are preferred. The status of barons varies. It can be a very high title or something of little consequence. It is definitely a noble title, however, and needs to be clearly distinguished from baronet.

Baronet

This may be thought of as a hereditary knighthood. For convenience, it may also be thought of as a noble title, though there are those who would disagree, at least as used in the British system. A baronet is certainly not a peer; in the United Kingdom, baronets are not entitled to a seat in the House of Lords (unless, of course, they additionally hold a peerage). The German Ritter when part of a last name, indicates the German equivalent of a baronetcy. A woman holding a baronetcy in her own right is termed a baronetess.

The Roman Era