After the HDMI cable, the next most popular cable these days is the digital optical cable. The reason for this is that it is now very common for users of Sky HD boxes to connect to a screen using an HDMI cable and then use an optical cable to send audio to an AV amplifier.
The key advantage of the optical cable is that unlike conventional cables, it is immune from interference. This is because the electrical signal is converted to a light signal that passes along reflective tube within the cable.
This also why optical cables need to be treated with care and not bent sharply as the tube can become damaged.
Many people assume that all optical cables sound the same since they are digital. However, we can confirm based upon our own evaluations that there are noticable differences in quality between different cables.