RS232 is a standard for serial connections. It uses pin 7 for ground, and pins 2 and 3 for transmit and receive. Other pins are used for hardware flow control, Ring indication etc. In theory there are two ends to a RS232 connection, a DTE and a DCE.
RS232 requires you to select a transmit/receieve rate called a baud rate. PCs commonly support up to 115200 baud. Almost everything defaults to 9600 (although not always).
Perry's helpful discovery for debugging serial connections:
<Remosi> It doesn't matter what baud rate you choose, if it's not plugged in
See mouse(4) for an example of low level driver details, and see the 
Serial HOWTO for a very detailed background and discussion.
See http://www.camiresearch.com/Data_Com_Basics/RS232_standard.html for a history of the standard.
