Here is the approximate pecking order of the English caste system,
compiled from various sources.

 I. SOVEREIGNTY
      king, queen
      prince, princess
II. SUBJECTS
    A. ECCLESIASTICAL (CLERGY)
    B. STATES
       1. MILITARY
          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'.
          Colonel: an officer commanding a 'column' of soldiers and support trains. 
          Captain: an officer entrusted with a command or fort under a sovereign or general. 
          Lieutenant: an officer representing and exercising powers on behalf of his lord or
               sovereign. 
          Sergeant: servant; attendant upon a knight in the field. 
          Corporal: influenced by 'corps'; head, chief. 
          Private: having attained no title of rank, a 'private soldier'. 
       2. MARITIME
       3. CIVIL
          a. Nobility (peerage, sometimes called "high nobility")
               i.  Duke
              ii.  Marquis
             iii.  Earl, Count
              iv.  Viscount
               v.  Baron
                     lord or nobleman; the most general title of nobility in England
                     judge of the court of exchequer
                     vassel holding directly from the king
          b. Dignity (degrees of honor, sometimes called "low nobility" though not nobility)
               Knights Baneret, created by sovereign in person on field of battle
                 Can lead vassels into battle under his own banner.
               Knight (not hereditary), a soldier, assistant to a superior 
                 commonly in return for land, "sir", a mounted man of arms serving a superior
                 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
                 Knights Baneret, created by sovereign NOT in person on field of battle
                    can lead vassels into battle under his own banner.
                 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
                 Knights Bachelors (the lowest, but most ancient of the ranks of knight)
                 Knights of the Chamber (title awarded in sovereign's chamber in peacetime)
               Esquire
                 title of office for sheriffs, serjeants, barristers at law, justices, 
                 and others
               Gentleman
                 One without title, but with a coat of arms showing ancestry
                 A person of superior birth, above a yeoman.
               Yeoman
                 yeoman, freeholder, a man freeborn, butler for nobility, 
                 gentleman attendant in royal household, "young man"
            c. Peasant
                 serf, countryman, tiller of the soil, laborer
Other terms
   NOBILITY, depends on context.  If no discernable 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.  Washington