High-Dynamic-Range (HDR) is a technique used in photography to reproduce a greater dynamic range of luminosity (e.g. exposure & brightness of an image) than is possible with standard photographic techniques. The aim is to present the human eye with a similar range of luminance as that which, through the visual system, is familiar in everyday life. The human eye, through adaptation of the iris (and other methods) adjusts constantly to the broad dynamic changes in our environment. The brain continuously interprets this information so that most of us can see in a wide range of light conditions. Most cameras, on the other hand, cannot.
I put together the slider below so you can see the original image that the camera thought was ideal versus the image compiled using data from three bracketed images and brought together using special software (Photomatix). Note that the camera tried to balance the bright light coming from the two windows with the shadow areas of the until room. This range of light is hard for the camera to capture so it finds a spot with the least amount of luminance compromise.