Serial vs. Parallel

The fundamental difference between the two formats is how the data is transferred between the device and the processors.
Traditional ATA devices and controllers use a parallel data transfer mechanism. Parallel processing is a fairly common technique
where multiple channels of data are sent simultaneously to try and increase the amount of data transferred in a single clock cycle.
In the case of the ATA/100 standards used by today's IDE drives and controllers, they send the data across a 16-bit channel.
The problem with this type of mechanism is the number of wire required to transfer that data. This is why the ATA cables are so wide.
It is necessary to have the 40 or 80 wires required to transfer the data. The problem with this is the interference caused between these wires.
At higher clock speeds necessary for faster speeds, the interference between the wires is too great to allow for reliable transmission.

Over the last couple of years, many advances have been developed in serial transmission techniques.
Specifically through the development of the Universal Serial Bus interfaces.
Serial transmissions run across a single control channel compared to the multiple channels of a parallel interface.
This means that at the same clock speeds, the serial line will carry less data, but because the serial method requires fewer wires,
less interference is generated to cause data integrity problems. This allows for serial transmission methods to run at much higher speeds
than the equivalent parallel methods.   In the case of the first Serial ATA standard, the clock runs at 1500 MHz compared to a clock rate of 50 MHz of the ATA/100 standard.