Which language uses .pde extension?

Bad news I'm afraid (or maybe great news?) : it isn't C code, it's an example of "Processing" - an open source language aimed at programming images. Take a look here

Looks very cool.

This code is from Processing.org an open source Java based IDE. You can find it Processing.org. The Arduino IDE also uses this extension, although they run on a hardware board.

EDIT - And yes it is C syntax, used mostly for art or live media presentations.

The .pde file extension is the one used by the Processing, Wiring, and the Arduino IDE.

Processing is not C-based but rather Java-based and with a syntax derived from Java. It is a Java framework that can be used as a Java library. It includes a default IDE that uses .pde extension. Just wanted to rectify @kersny's answer.

Wiring is a microcontroller that uses the same IDE. Arduino uses a modified version, but also with .pde. The OpenProcessing page where you found it is a website to exhibit some Processing work.

If you know Java, it should be fairly easy to convert the Processing code to Java AWT.