DVI-A is a high-def analog signal. DVI-A connectors have the four pin connections around the slotted connector (on the right half of this image).
The conversion from digital to analog occurs in the video card. Technically the overall image quality it is much higher quality than standard VGA. But the reality is, very little difference can be seen with standard definition displays/projectors (eg. 1024×768 or 1280×1024).
The real advantage of DVI-A is its flexibility with hardware connections. It can be connected to DVI devices, or easily converted to a VGA connection with a DVI-to-VGA adaptor (approx. $10).
– Troy @ TLC