For many folks, Processing is something new. For illustration, here is a progression of easy sketches that give some of the flavor of Processing.
This sketch opens a window, and wherever you move the mouse, it draws an ellipse. The
draw() method fires 60 times per second (i.e. the frame rate is 60 fps). You can play with it yourself, or just watch the video.
Here's a video demonstration of the sketch:
This sketch introduces color using RGB. In Processing, you specify a value from 0-255 for each of the three primary light colors. Zero means the color is excluded, 255 means you are using a maximum amount of that color.
Color can also have a transparency level. In this sketch, we let our ellipse fade out by drawing a faint white rectangle on top of our drawing. At 60 fps, the paint gradually disappears.
You can detect how fast your mouse is moving. This sketch uses that information to change the behavior of the drawing.