Play with this Bézier Curve by adjusting the endpoints (circles) and the handlebars (squares).
Bézier Curves are mathematically defined curves that play an important role in computer graphics because of their ease of manipulation. For more information on them, visit this link: http://en.wikipedia.org/wiki/Bézier_curve (I find the animated GIFs under the section "Constructing Bézier curves" especially useful).