The Collaborative International Dictionary of English v.0.48:

Folk \Folk\ (f[=o]k), Folks \Folks\ (f[=o]ks), n. collect. & pl.
   [AS. folc; akin to D. volk, OS. & OHG. folk, G. volk, Icel.
   f[=o]lk, Sw. & Dan. folk, Lith. pulkas crowd, and perh. to E.
   follow.]
   1. (Eng. Hist.) In Anglo-Saxon times, the people of a group
      of townships or villages; a community; a tribe. [Obs.]
      [1913 Webster]

            The organization of each folk, as such, sprang
            mainly from war.                      --J. R. Green.
      [1913 Webster]

   2. People in general, or a separate class of people; --
      generally used in the plural form, and often with a
      qualifying adjective; as, the old folks; poor folks.
      [Colloq.]
      [1913 Webster]

            In winter's tedious nights, sit by the fire
            With good old folks, and let them tell thee tales.
                                                  --Shak.
      [1913 Webster]

   3. The persons of one's own family; as, our folks are all
      well. [Colloq. New Eng.] --Bartlett.
      [1913 Webster]

   Folk song, one of a class of songs long popular with the
      common people.

   Folk speech, the speech of the common people, as
      distinguished from that of the educated class.
      [1913 Webster]

WordNet (r) 3.0 (2006):

folk
    n 1: people in general (often used in the plural); "they're just
         country folk"; "folks around here drink moonshine"; "the
         common people determine the group character and preserve
         its customs from one generation to the next" [syn: folk,
         folks, common people]
    2: a social division of (usually preliterate) people [syn:
       tribe, folk]
    3: people descended from a common ancestor; "his family has
       lived in Massachusetts since the Mayflower" [syn: family,
       family line, folk, kinfolk, kinsfolk, sept,
       phratry]
    4: the traditional and typically anonymous music that is an
       expression of the life of people in a community [syn: folk
       music, ethnic music, folk]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "folk":
   Everyman, John Doe, Public, acknowledged, admitted, animal kingdom,
   blood, body politic, breed, brood, citizenry, clan, class,
   common man, commonwealth, community, community at large,
   constituency, conventional, cultural community, customary, deme,
   dwellers, established, estate, ethnic group, everybody, everyman,
   everyone, everywoman, family, fixed, folks, general public, gens,
   gentry, habitancy, hallowed, handed down, heroic, hoary, house,
   household, immemorial, inhabitants, inveterate, kind, kindred,
   legendary, line, lineage, linguistic community, long-established,
   long-standing, matriclan, men, menage, mythological, nation,
   nationality, of long standing, of the folk, oral, order, patriclan,
   people, people at large, people in general, persons, phratry,
   phyle, plant kingdom, polity, populace, population, prescriptive,
   public, race, received, recognized, rooted, sept, society, species,
   speech community, state, stem, stirps, stock, strain, time-honored,
   totem, traditional, tribe, tried and true, true-blue, understood,
   unwritten, venerable, whole people, world, worshipful,
   you and me