Bezier curves are a neat little thing in computer graphics which allow you to define the shape of a curve just by specifying a few control points. In this program, you can see how control points affect the curve by clicking on them with your mouse and dragging them around.
If you don't see anything on the screen within a few seconds, just click on the screen anywhere and drag your mouse around. It is designed to repaint when you click, as a way of covering up for a minor display bug.
For more information on what Bezier curves and surfaces are and what they are good for, you ought to check out this great article
.Click here to see the code.