DVI-D cables also know as true digital video cables and are used for a true digital connection between two electronic components (normally a video card and an LCD monitor). All video cards ever created have always been able to create a digital output, the only downside before DVI came about was that this output had to be downgraded to analog before being sent to the monitor - where it was then converted back to digital by the monitor! Using a DVI-D cable means this process isn't required and therefore the end result is a very quick and extremely high quality visual output.
DVI-A cables also known as high resolution analog cables and are used to transmit a DVI signal to an analog display unit (normally a CRT monitor and becoming more used on HD televisions). Although this type of output isn't as good as DVI-D (due to the loss of data in the digital to analog conversion processes), it still gives a much improved output than the display from a standard VGA output.
DVI-I cables are capable of transmitting between two devices in either a digital to digital format or an analog to analog format, but (and that's a big BUT) it will not connect two dissimilar devices (i.e. digital to analog or analog to digital).
Note: DVI digital and DVI analog formats are not interchangeable, basically this means that a DVI-A cable will not work on a digital system and a DVI-D cable will not work on an analog system. You must know before purchasing a DVI cable what format your two devices are, remember that only equipment with a DVI port labeled DVI-I will accept both a DVI-A and DVI-D signal.
So how can you tell what type of DVI cable you currently have? The easiest way is to read the manual for the devices you are connecting, or by looking at the DVI cable. There are two ways to determine what DVI format a device is by looking at the end of the DVI cable face on: 1. There is always a flat pin on one side of the DVI cable, a flat pin alone denotes a DVI-D connector and a flat pin with four surrounding pins can be either a DVI-I or DVI-A connector. 2. The pins on the DVI cable denote whether the cable is single or dual link, 24 pins in 3 rows of 8 means it's a DVI dual cable, while two lots of 9 pins with a gap in the middle means you have a DVI single link cable. Note: To work out if a cable is DVI-I or DVI-A you need to look at the number of pins on the DVI cable end, 24 pins means you have a DVI-I cable and a set of 8 pins with a gap to a set of 4 pins denotes a DVI-A cable.