RS232 is a standard for serial connections. In theory there are two ends to a RS232 connection a DTE and a DCE.
RS232 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.
RS232 requires you to select a transmit/receieve rate called a baud rate. "Standard" PC's support up to 115200 baud. Almost everything defaults to 9600 (although not always).
See mouse(4) for an example of low level driver details, and see HowToSerialHOWTO? for a very detailed background and discussion.