It depends a lot on in which country and at what time you are referring to.

Earl and Count are the same level of nobility - what was called Greve in Sweden, Graf in Germany and Comte in France was called Earl in England. The Earl was adressed as Lord, just as a Knight was adressed as Sir. The wife of an Earl was referred to as a Countess rather than an Earless, though.

A Duke is a rank above Count/Earl. Depending on the size of the country, the rank of Duke could be reserved for the ruling family (in Sweden, for example, the only Dukes/Duchesses are the siblings of the monarch and their spouses) or have independent families as the rulers of a Duchy - these were often the descendants of a sibling of a monarch, though.

Generally, here's a list of nobility and rulers with some explanations.

• Emperor.

• King.

• Prince. This is not a Prince as a son of a monarch, but rather an indpendent ruler that is not a King. The electors of the Holy Roman Empire often carried this title (Kurfürst). The title for the British heir to the throne, Prince of Wales comes from being Prince of the Princedom of Wales rather than being a Prince as in the son of a King (which the heir also is).

• Duke. There are many variations on this to signify a higher rank. Russian Grand Dukes (Velky Knyaz) could be translated as Grand Prince, but is usually translates as Grand Duke in English-language sources. There's also the Archduke (Erzherzog) of Austria and the Grand Duke (Grossherzog) of Luxemburg.

• Marquis or Markgraf, originally a title for a Count that had a larger degree of independence since he guarded the border far away from the centre of the realm.

• Count (and Earl).

• Viscount.

• Baron.

• Baronet.

• Knight.

• Esquire.

if you're interested in origins –

Count – count is the continental counterpart of earl, sort of a mid-level of nobility, ranking above a baron and below a duke, first used by the Normans. Comes from a Latin word comes for a Roman general commanding two centuries of soldiers (not a hereditary title in the Roman Empire). As a hereditary title of nobility in the Middle Ages, counts were appointed generally by a duke or directly by a monarch, the latter case being called a count palatine.

Duke – in a duchy the duke is the ruling monarch (this could be called Grand Duke or as the Hapsburgs came to be titled, Archduke), in eg. a kindgom or a principality, he is the highest-ranking noble below the monarch. In some continental traditions that do not reserve the title of prince exclusively for legitimate children of the monarchy, a grand duke could alternatively be titled prince or Grand Prince. Herzog in German, duce in Italian (think of Mussolini's moniker, Il Duce).

derived from the Latin dux which was, again, a military rank originating in Imperial Rome the Roman Republic which was originally an informal term for a general but during the time of the Empire came to mean the commander of an expeditionary force.

Lord – in Britain is still an appellation for any (male) member of the hereditary peerage, really just a general feudal term for a landowner (think of the word "landlord").

Earl – Britain and Ireland's equivalent of the continental count, derived from the Scandinavian jarl which could mean anything from a local chieftain to a sovereign prince. Pretty interesting fact – Britain is believed to have retained the traditional Anglo-Saxon title of earl even after the Norman invasion because the Norman count sounded too much like the Old English expletive cunt.

if you're interested there is also baron, viscount, marquis, and of course prince.

To add on to /u/vonadler's excellent example (and again, this is dependent on the country in question), "lord" (or "lady") is a way to refer to members of the hereditary nobility, so anyone with a certain type of noble title -- a peerage, to be specific. In /u/vonadler's example, a "lord" would be anyone above the rank of baron, but you wouldn't call a knight or an esquire a "lord." (Confusingly, lords could also be granted knighthoods, but knighthood is not a heritable title.)

Here is how titles of nobility developed in England. The development was somewhat different in other parts of Europe.

The titles of Medieval British nobles varied depending on what part of the Middle Ages you are looking at.

Before the Norman conquest there were Earls in England and Thanes in Scotland and these were the only titles of nobility other than Royalty.

William the Conqueror introduced the title of Baron to England, and he kept the title of Earl. For quite a long time, these were the only two titles held by English nobility (other than the title of Duke of Normandy, claimed by the English Kings).

There were generally less than 200 Barons in England in the period between 1066 and 1300.

In 1307, at the end of the reign of Edward I, there were eleven Earls in England and Ireland.

The title of Duke was first created in England by Edward III in 1337 when he named his eldest son, Edward, the Black Prince, Duke of Cornwall. By 1483, there were 16 Ducal titles in England and Ireland.

The title of Duke outranked all other titles except King and Queen and Prince of Wales in England.

The first Marquess in England (above an Earl but below a Duke) was Richard de Vere, 9th Earl of Oxford, who was appointed Marquess of Dublin by King Richard II in 1385. He was later created Duke of Ireland, and the title of Marquess was eliminated. In 1397, John Beaufort, 1st Earl of Somerset was appointed Marquess of Dorset, but he fell out with the King, and the title was revoked in 1399. There were no other Marquesses created until 1442.

The title of Viscount (below an Earl but above a Baron) was created in 1440 when John Beaumont was created Viscount Beaumont by King Henry VI. This remained an uncommon title.

The most powerful leader of the Welsh realms was called the Prince of Wales (by the English) from about 1165. (Tywysog Cymru, in Welsh, which means "Leader of the Welch".) In 1301, Edward I after conquering Wales, gave the title to his eldest son, so the title "Prince" (of Wales) became an English title, not just an honorific for the sons of the monarch.

For most of the Medieval period, the only titles of English nobles were Earl and Baron. Between the conquest and about 1300, these titles usually implied rights over lands and a feudal duty to provide military support to the King.

In the later Middle Ages, the new titles of Duke, Marquess and Viscount first appeared. These titles appeared as the relationship between titles and land was diminishing and the titles were more of a recognition or an honor than part of civil or military administration. (Though, of course, title holders were generally rich and powerful men, and had a role through Parliament (if one was called) in civil administration, just not a role in administering civil judicial or governmental duties in regions of the Kingdom.)

The title of Duke became fairly common, particularly to bestow upon the sons of the Monarch (though it died out again after the Middle Ages. After Queen Elizabeth I executed the Duke of Norfolk in 1572, there were no Dukes left in England - though the title was re-instated, and there are now 30). The titles of Marquess and Viscount remained fairly sparsely used in England, certainly in Medieval times.