The retina display is a liquid crystal display technology that Apple uses in the iPhone, iPod Touch, iPad and Macbook Pro. It is supposed to be such a high pixel density screen that the human eye cannot notice the pixels at a normal viewing distance.

The current iPad has a 2048 X 1536-pixel resolution screen which is made up of 3.1 million pixels that are shown at 264 pixels per inch. Each of these pixels have a multiple signal that prompts it to light up, however they are separated from the signals in a way to make sure the image remains crystal clear.

Source: iPad Design