Standard PC serial ports come in to versions: 9 pin and 25 pin one. The functions of those both version are exactly the same, only different kind of connectors and different pinout. If you need to convert from one version to other you can do it easily just bu buying a suitable adapter from your local computer shop. PC serial port is nowadays usually used for interfacing your PC to your modem or mouse. Original PC serial port was designed to operate up to 19.2 kbit/s (maximum speed defined in RS-232C standard) but nowadays they can typically go up to 115.2 kbit/s (some special cards can do even faster than that).
PC parallel port was originally desingned for connecting your PC to a parallel printer which has Centronics interface. Nowadays the this same port is used for interfacing lots of different kind of peripherals to PCs (scanners, Zip-drives, backup tapes, network adapters). To make the interfacing more efficent the technology has evolved during years. Nowadays paralle port can handle many different operating modes like SPP, ECP and EPP.
IrDa is an infrared data link standard for short distances. First versio runs uo to 115.2 kbit/s but newer versions can handle speeds up to 4 megabits/s. Many modern PC motherboards have nowadays interface for IrDA module (IR transmitter and receiver). There is some general information on IrDA interface below, including pinouts of IrDA connectors in some motheraboards. What lacks is a schematics of suitable IrDA module which could be connected to PC motherboard with IrDA connector.
USB is a serial bus standard for interfacing many different kinds peripherals to your PC. It is aimed to replace most of the different kind of special systems of interfacing external devices to serial and parallel port. Most of the new PCs or motherboard have had USB connectors for some time, but the system has not catched mass markests yet because of lack of drivers (Windows 98 has those). USB can power to the devices connected to it. USB can provide plug and play with hot swapping capabilities. USB provides 12 Mbit/s and 1.5 Mbit/s data transmission speeds. USB can't be used directly as a fast connection between PCs, but here are special products which allow small scale networking using USB bus.
IEEE 1394 is a fast serial bus which is a little bit similar to USB but is capable of much higher speeds (upt to 400 Mbit/s). IEEE 1394 is nowadays uded for interconnecting modern digital video equipments to PC. Allows live connection/disconnection. IEEE 1394 was called Firewire before standardization.
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