Processing is an open source language/ development tool for writing programs in other computers. When you want those other computers to “talk” with an Arduino, for instance to display or save some data collected by the Arduino. Arduino comes with some basic examples for communicating with Processing (in Examples > Communication). If you just want to control an Arduino board from a Processing program, you may want to use the Arduino library for Processing described below. First you upload a standard firmware (program) to the board and communicate with it using the library. The firmware is called Firmata, and is included in the Arduino software. The corresponding Processing library can be downloaded below.