Arduino does not only support RS-232 serial communication, but also supports two more communication protocols:
Serial Peripheral Interface (SPI): A special master/slave serial communication protocol used in short distance communications and very popular among different types of sensors. Its main disadvantage is the need for four pins to establish the communication channel.
Inter-Integrated Circuit (I2C): A bus-based master/slave communication protocol allowing for multimasters and multislaves. It is mainly aimed at connecting low speed devices.
They are out of the scope of this book and I will simply give you some links in case you want to investigate a little more by yourself:
Regarding SPI, you can read its Wikipedia page at http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus
To know how to deal with SPI from Arduino, there is a special dedicated page to this protocol at the Arduino Reference section of the Arduino site at http://arduino.cc/en...