What exactly is HDR?

HDR=High Dynamic Range Imaging. It basically involves taking the light areas and dark areas of an image and darkening/lightening them by using a darker or lighter version of an image to form a final image where you can see the highlights, midtones, and shadows. Typically it involves taking a few images in a bracketed (multiple images from light to dark) setting. As always, the wiki does a far better job of explaining than I ever will http://en.wikipedia.org/wiki/High_dynamic_range_imaging.