Answer:
   
   In classic Halachic literature, Shabbat begins at sundown. The 18
   minute custom arose for various reasons that include the following:
     * There is a mitzvah to add to the shabbos by beginning it early and
       ending it later.
     * Time pieces are imprecise.
       
   As a result of this, the custom developed to light candles some
   specific amount of time before sundown. In the United States, the 18
   minute custom was almost (but not quite) universally accepted because
   the first printed calendars in the US were printed by Rav Henkin, and
   marked candle lighting 18 minutes before sundown. Most calendars in
   most cities in the US follow that format today.
   
   Note: While one can/should bring Shabbat in early with the lighting of
   candles, Shabbat begins at sundown even if candles have not been lit.
   For traditional Jews, at that point candle lighting would be
   forbidden.
   
   Note that other cities may have different customs. In Jerusalem, the
   custom seems to be to light candles 30 minutes before sundown. In
   Chicago Illinois, the custom is 20 minutes.