Answer:
   
   The prohibition is phrased in the Talmud as "voice, in a woman, is
   something erotic". In Aramaic, "qol be'ishah ervah" (from which comes
   the common name for the prohibition, "kol ishah") With the leading
   "be-" (in) omitted, it means "a woman's voice".
   
   The fundamental prohibition is on men--that they are not to listen to
   women sing. There is a law, though, against causing others to sin. It
   comes from the verse "Do not place a stumbling block before the
   blind." Therefore, implied in a man's prohibition against listening is
   a woman's against singing in a situation where men would be listening.
   
   However, in practice, there are leniencies. For example, it does not
   apply to immediate family members. Most rule it does not apply to
   recorded or remotely transmitted voices. Many rule it does not apply
   to sung prayer. Some rule it does not apply to group singing, only
   when a woman sings alone. Different communities have different
   practices.