The main purpose of the Arduino Serial USB board is to allow the Arduino Mini to communicate via USB to your computer, see Connect to Arduino Mini.

The Arduino Serial USB Board shows up as a com port allowing serial communication to and from the Arduino Stamp or any other serial device. The Arduino Serial USB adapter uses the FT232RL which is a USB to serial UART (Universal Asynchronous receiver/transmitter) interface.

Arduino is an open-source physical computing platform based on a simple i/o board and a development environment that implements the Processing/Wiring language. Arduino can be used to develop stand-alone interactive objects or can be connected to software on your computer (e.g. Flash, Processing, MaxMSP). The open-source IDE can be downloaded for free (currently for Mac OS X, Windows, and Linux).