Bezier curves
One of the toughest concepts to master regarding drawing via computer is the Bezier (bez-ee-ay) curve. Created by French mathematician Pierre Bezier to streamline the process of cutting car parts with automated machines, Bezier curves provide an inventive way to control curves. Using two endpoints, or anchorpoints, and two control handles for each line segment, you can create a wide variety of curves.
Unfortunately these curves are one of the least intuitive aspects of drawing ever to be loosed upon unwitting artists. The learning 'curve' is so steep it seems unclimbable at first. Eventually, after some practice, the magic of Bezier curves become apparent.
(Source: The Macintosh Bible, 7th Edition, page 720)