Answer:
   
   Amongst Ashkenazic Jews, this custom is called upsherin, a Yiddish
   word from the same root as the English "shearing". Upsherin is
   prevalent in Chassidic and Sepharadic communities, communities that
   share a number of Kabbalistically derived custom.
   
   The more popular reason given for the custom is the law of "arlah",
   that one may not use fruit from a tree until its fourth year. In
   explaining the prohibition against needlessly killing fruit trees, or
   wastage in general, the Torah (Deut) uses the expression "for a man is
   a tree of the field" (what you waste now could cost lives later). This
   expression could be taken to compare people with trees. Since the
   "tree of the field" isn't harvested for three years, neither do we cut
   a boy's hair.
   
   But why just boys?
   
   Those who keep the custom of upsherin also wait until the child turns
   three before giving him a yarmulka and tzitzis to wear. Also, the day
   of upsherin the child is taken to a teacher and shown the alef-beis.
   In "the old country" the child would start cheder (single-classroom
   school), now this formalized "start of education" is done instead.
   This gives an explanation about why boys in particular.
   
   The obligation to educate girls in Torah is functional--you can't be a
   good Jew without knowing Judaism. However, for boys there is in
   adddition an obligation to study Torah as an end in itself. Since
   upsherin marks the start of the mitzvah of education, it's therefore
   tied to gender. The educational aspect also adds a second layer of
   meaning to the custom. The prohibition of using fruit of a young tree
   is called "arlah". The same word used for an uncircumcised foreskin.
   Circumcision is commonly used as a symbol of removal of barriers; both
   in Jewish tradition, and in Paul's letters where he writes of
   "circumcision of the heart". Here we see "circumcising" the mind and
   head, removing the "arlah", as a preparation for schooling. Add to the
   change in self image of the haircut and starting to wear tzitzis and
   yarmulka, and upsherin becomes a rite of passage from babyhood to
   childhood.