If you pay attention to it, all technologies that exist today are migrating from parallel communication to serial communication.  The new IDE standard for hard disks is serial (ATA Serial).  The PCI bus will be transformed into serial in the years to come, with the release of its new version, the PCI Express.  The SCSI interface is also being transformed into serial.

The serial communication differs from the parallel one for only transmitting a bit at a time, while in the parallel communication several bits are transmitted per time.  That makes the parallel communication faster than the serial one.

That statement, however accepted by most people, is not totally true.  The serial communication may be faster than the parallel one, all you need is that the bits leave the transmitting device at a much higher speed.  An example of such is the ATA Serial port that however serial can reach a transfer rate of up to 150 MB/s, while the traditional IDE port gets to reach 133 MB/s at the most.

There are several reasons to make the devices migrate from the parallel communication to the serial one.  In the parallel communication, since several bits are transmitted per time, a wire is required per each bit.  For instance, in a 32 bit communication (as it is the case of the PCI slot) 32 wires are required just for the data transmission, not to mention the additional control signals that are necessary.  The higher the quantity of bits being transmitted per time, the more wires are used, making the creation of cables and the construction of boards difficult. In the serial communication, only two wires are required, making it much easier to project the communication between two devices.

The higher the transfer rate, the bigger the problem with the electromagnetic interference.  Each wire becomes an antenna in potential, capturing a lot of noise from the environment, which may corrupt the data transmitted.  In the parallel communication, since many wires are used, the problem of the electromagnetic interference is a serious one.  In the serial communication, on the other hand, since only two wires are used, that problem is much more easily solved, by simply protecting the two wires used.

There is yet another problem, a not much discussed one.  Even though we say that in the parallel communication all the bits are transmitted at the same time, the bits do not get to the receiver exactly at the same time.  If in low performance devices the small time difference in the reception of the several bits of data is not important, in high-speed devices that difference in the reception time of the several bits makes the device wastes time having to wait for all the bits to arrive, which may represent a significant fall in performance, since the data transmission operation happens in very short times.

Another difference between the parallel communication and the serial one is that the parallel communication is half-duplex, while the serial one is full-duplex.  In plain English, that means simply the following:  in the parallel communication, the only path between the transmitter and the receiver is used both for the transmission and for the reception of the data.  Since there is only one path, it is not possible to transmit and receive data at the same time.  In the serial communication, on the other hand, since it only uses two wires, the manufacturers usually make four wires available, two for the transmission and two for the reception of data.  In other words, a path just for the transmission of data, and another one only for its reception. That makes it possible for the simultaneous transmission and reception of data. Such architectural difference alone makes the serial communication potentially twice as fast as the parallel communication, if we compare two communications that have the same transfer rate.