Nobility 

​Here is the approximate pecking order of the English caste system, compiled from various sources.
  1. The SOVEREIGNTY
    1. King, Queen
    2. Prince, Princess
  2. ​The SUBJECTS
    1. ​ECCLESIASTICAL (Clergy)
    2. STATES
      1. MILITARY
        1. General: originally meant to be of similar 'birth' or 'class' with the sovereign; the more recent use is to be familiar with all facets of the army, no longer a specialist in one area, a 'general officer'.
        2. Colonel: an officer commanding a 'column' of soldiers and support trains.
        3. Captain: an officer entrusted with a command or fort under a sovereign or general.
        4. Lieutenant: an officer representing and exercising powers on behalf of his lord or sovereign.
        5. Sergeant: servant; attendant upon a knight in the field.
        6. Corporal: influenced by 'corps'; head, chief.
        7. Private: having attained no title of rank, a 'private soldier'.
      2. ​MARITIME
      3. CIVIL
        1. ​Nobility (peerage, sometimes called 'high nobility')
          1. ​​​Duke
          2. Marquis
          3. Earl, Count
          4. Viscount
          5. Baron, lord or nobleman; the most general title of nobility in England; judge of the court of exchequer; vassel holding directly from the king
        2. ​Dignity (degrees of honor, sometimes called 'low nobility' though not nobility)
          1. ​Knights Baneret, created by sovereign in person on field of battle; can lead vassels into battle under his own banner.
          2. Knight (not hereditary), a soldier, assistant to a superior commonly in return for land, "sir", a mounted man of arms serving a superior.
            1. Knights of the Garter, aka Knights of the Order of St. George Baronet, granted by patent, lowest hereditary dignity or degree of honor but not a title of nobility, baronets are commoners.
            2. ​​Knights Baneret, created by sovereign NOT in person on field of battle; can lead vassels into battle under his own banner
            3. Knights of the Bath (took a bath the night before his creation); The order originally consisted of the sovereign, grand master, and 36 knights companion
            4. Knights Bachelors (the lowest, but most ancient of the ranks of knight)
            5. Knights of the Chamber (title awarded in sovereign's chamber in peacetime)
          3. ​Esquire; title of office for sheriffs, serjeants, barristers at law, justices, and others.
          4. Gentleman; One without title, but with a coat of arms showing ancestry; A person of superior birth, above a yeoman.
          5. Yeoman; freeholder, a man freeborn, butler for nobility, gentleman attendant in royal household, "young man".
        3. ​Peasant;  serf, countryman, tiller of the soil, laborer.
Other terms:
NOBILITY, depends on context.  If no discernible context or a legal context, then "nobility" only refers to duke, marquis, earl, viscount, and baron.  In a non-legal context lower ranks are sometimes called low nobility while the upper ranks are called high nobility.  Sometimes "nobility" includes both high and low nobility.

SQUIRE short for esquire

GENTRY of noble birth

GENTLEMAN can refer to all of the nobility; Its meaning depends on the context in which it is used.

PEER a nobleman; as a peer of the realm; the House of Peers, so called because noblemen and barons were originally considered as the companions of the king. In England, persons belonging to the five degrees of nobility are all peers [members of the peerage].

CITIZEN
  1. The native of a city, or an inhabitant who enjoys the freedom and privileges of the city in which he resides; the freeman of a city, as distinguished from a foreigner, or one not entitled to its franchises.
  2. A townsman; a man of trade; not a gentleman.
  3. An inhabitant; a dweller in any city, town or place.
  4. In a general sense, a native or permanent resident in a city or country; as the citizens of the United States.
  5. In the United States, a person, native or naturalized, who has the privilege of exercising the elective franchise, or the qualifications which enable him to vote for rulers, and to purchase and hold real estate.  
If the citizens of the United States should not be free and happy, the fault will be entirely their own.  -G. Washington
"Through the courts, encouraging the government to obey the law."