Hopefully you know the difference between digital and analog signal transmision. DVI is short for Digital Video Interface. DVI-I stands for "DVI-Integrated" and supports both digital and analog transfers, so it works with both digital and analog monitors. DVI-D" stands for DVI-Digital that supports digital transfers only.
The DVI interface uses a digital protocol established throughout the industry. The desired illumination of pixels is transmitted as binary data. In native resolution, DVI will read each number and apply that brightness to the appropriate pixel. In this way, each pixel in the output buffer of the source device corresponds directly to one pixel in the display device. With Analog signals, the appearance of each pixel may be affected by its adjacent pixels, by electrical noise, and some other unpredictable forms of analog distortion.
Previous standards such as the analog VGA were designed for Cathode Ray Tubes, or CRT-based devices. When the analog source transmits each horizontal line of the image, it varies its output voltage to represent the desired brightness. In a CRT screen, this is used to vary the intensity of the scanning beam as it moves across a screen.
DVI cable connectors do not allow the user to connect the cable in an incorrect position or orientation. In the digital transfer one or two channels are present. Video and monitor cards which are exclusively digital cannot be connected to analog. They can, however be connected to equipment that can control both analog and digital signals.