Processing is a programming environment for beginners interested in doing interactive graphics in Java. The website says that:
Processing is a context for exploring the emerging conceptual space enabled by electronic media. It is an environment for learning the fundamentals of computer programming within the context of the electronic arts and it is an electronic sketchbook for developing ideas.
which sounds rather grand. Technically speaking, Processing is Java plus a cute, easy IDE that lets you do everything that is necessary with a minimum of fuss, plus a very sweet graphics library called bagel that is similar to OpenGL but is not based on OpenGL.
Processing has a small but very active community of users, and an excellent website at http://www.proce55ing.net/ - notice that those are fives, not S letters.
The development of Processing rests on the shoulders of Casey Reas and Ben Fry, and most of the work has been done at the MIT and Interaction Design Institute Ivrea.