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 for a history of the standard.