DVI and HDMI transmit video signals in a digital format as opposed to the traditional analog VGA/SVGA format.  Digital transmission technology results in a superior quality image over analog transmission technology due to analog's sensitivity to the phase changes of cables.  This is also the reason DVI and HDMI are better than component video.

The only drawback of digital transmission technology is that the data being transferred can only maintain full fidelity for about 5 meters with DVI and about 15 meters for HDMI.  Cable runs can be increased with the use of fiber optic technologies, amplifiers, and repeaters, however, their incredible expense makes the idea commonly unreasonable.