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):

folks
    n 1: your parents; "he wrote to his folks every day"
    2: 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]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "folks":
   agnate, ancestry, blood, blood relation, blood relative, brood,
   children, clansman, cognate, collateral, collateral relative,
   connections, consanguinean, distaff side, distant relation, enate,
   family, flesh, flesh and blood, german, get, hearth, homefolks,
   house, household, issue, kin, kindred, kinfolk, kinnery, kinsfolk,
   kinsman, kinsmen, kinswoman, kith and kin, menage, near relation,
   next of kin, offspring, people, posterity, relations, relatives,
   sib, sibling, spear kin, spear side, spindle kin, spindle side,
   sword side, tribesman, uterine kin