From the Processing website:

Processing is an open source programming language and environment for people who want to create images, animations, and interactions. Initially developed to serve as a software sketchbook and to teach fundamentals of computer programming within a visual context, Processing also has evolved into a tool for generating finished professional work.

Processing cross platform and is a totally free to download, use and publish with programming language. The download includes the programming environment and compiler as well. Processing is based on JAVA and can use many different JAVA libraries. It has also now has the ability to compile apps for Android as well as run as Java Script. It is these to latter functions that I focus on for this site.

By downloading the Android SDK you can expand the functionality of Processing to enable it to create applications for Android phones or tablets. These apps can be signed and sold in the android App market.

Processing.js allows your Processing projects to run as Java Script. All modern web browsers support Java Script, including browsers on mobile devices. Additionally, custom interactive widgets for iBooks 2.0 can be created using Java Script or now by extension Processing.