The "standard"* serial port on the Macintosh is an 8-pin mini-DIN female. The above image and notes below indicate the pin assignments of this interface. Notice that this really isn't RS232-C, but rather RS422, as the data signals come in balanced pairs, a positive (plus) and a negative (minus).
Output handshake (Zilog 8530 DTR pin)
Input handshake (DSR) or external clock (selectable)
Transmit data (minus)
Receive data (minus)
Transmit data (plus)
General purpose input (not on Mac Plus or earlier)
Receive data (plus)
RS449, RS422, RS423, and such...
RS449 was issued in 1975 and was supposed to supersede RS232-C, but few manufacturers have embraced the new standard. RS449 contains two sub-specifications called RS422 and RS423. While RS232-C modulates a signal with respect to a common ground (called single-ended transmission), RS422 modulates two signals against each other (called differential transmission). An RS232-C receiver senses whether the received signal is sufficiently negative with respect to ground to be a logical "1"; the RS422 receiver simply senses which line is more negative than the other. This makes RS422 more immune to noise and interference, and more versatile over longer distances. The Macintosh serial ports follow RS422, which can be converted to RS423 by proper wiring of an external cable. RS423 can then communicate with most RS232-C devices over distances of 15 meters or so.
A serial port is no longer standard on a Macintosh, and has been replaced with USB and FireWire ports.