Processing is an computer language stemming out ideas developed in the Aesthetics and Computation Group at the MIT Media Lab by Ben Fry and Casey Reas.
It was originally developed to help teach the fundamentals of computer programming has since grown into a powerful and easy to use language that has enabled many would be digital artists and designers to make programatic, real time, or data driven art and design.
Processing is built on Java by Sun Microsystems, who between 2006-2007 released nearly all of Java and open source under the GNU GPL (General Public License) (the bits they did not release, they did not hold the copyright to). Processing acts as a great gateway language. It is simple and easy to use, but has all the power and extensibility of Java underneath. And so, as a person becomes more confortable with Processing and programming in general, and as their projects become increasinly complex, they are able to tap into Java to complete the more complex tasks.
The Processing project represents the development of open source tools to create open source art work. While there isn't and can't be a requirement for projects written in Processing to be open source, it is strongly encouraged. When exporting a "Sketch" from the Development environment, by default includes a link to the source code and the Processing website. However, is certainly possible to develop closed source or commercial software with Processing or Java.
In many ways, the nature of learning to program lends itself to open source. Early programming examples are based on previous works, many people learn by taking code that someone else has written and modifying it do what they want. Many projects, art and otherwise, are built from gluing together libraries, programs, functions, classes, and code form various sources together.
Share and share alike.
Essentially, Fry and Reas have developed a platform for art work. The art lies in the creation of a platform. Few artists want to become full time programmers. Often it is a means to an end, a tool to be used. But this tool opens up huge potential for engaging the data filled world we increasingly find ourselves living in.
The problem that I have with a lot of digitally based art, is paradoxically the exclusionary nature of technology and the power structures that control access. Along with access comes the (frequent) prerequisite knowledge of technologies to appreciate or engage in the work. This is not a function of the programming language, but perhaps the way in which it is being used. Paintings about paint, digital art about digital technology.
As technology is introduced to art, or art to technology, the initial response is to develop projects that generally have that "gee whiz" factor. The trick, is to move beyond that.
Some more Processing links: