Social Class: The Nobility

King with globus cruciger and sceptre

King/Queen - The ruler of the land.
Prince/Princess - The children of the King/Queen. The Crown Prince, if there is one, is typically the heir to the throne.

The hierarchy of the remaining noble titles can be remembered with the mnemonic device:

"Did Mi'lord Ever Visit Bosworth Battlefield?"

Duke/Duchess - The lord ruling a Duchy or one granted the title ceremonially. Either way they are usually a member of the extended royal family. Beyond being ruled by a Duke a Duchy is like a Shire.
Marquis/Marchioness - A Marcher Lord. Similar to an Earl but with more power because of the risks required subjugating the Marches.
Earl/Countess - A baron that controls a Shire on behalf of the King/Queen.
Viscount/Viscountess - Similar to a High Sheriff but working for a Duke instead of the King. Originally non-heridatry the Viscount became a hereditary title over time.
Baron/Baroness - A hereditary title, typically the holder of one or more fiefs.
Baronetcy/Baronetess - A hereditary title but not a Peer (and thus not entitled to attend the House of Lords).

Well equipped Men-At-Arms

Beneath the hereditary Lords are:
Knight - One granted the honorary title of knighthood by a king/queen, bishop or other political or religious leader for military service.
Man-At-Arms - A term that applies to all military folks, typically mounted and well equipped, despite their title. A Mercenary peasant and Countess armed for battle could both be called a Man-at-Arms despite title or gender although in medieval history female Men-At-Arms were more than a bit rare.