High Dynamic Range, or HDR refers to a technique of taking a series of images, combining them, and adjusting the contrast ratios to do things that are virtually impossible to get from a standard, single-exposure photograph. Think of it as stacking several differently-exposed photos of the same scene on top of each other, and using only the parts of each photo that are properly exposed. That's the simplified version...

If you've ever taken a photo indoors in the daytime with sunlight coming in through a window, you know that you can either expose for the interior of the room (and have the windows blown out white), or expose for the light coming through the windows, and have the rest of the room as dark as a cave. HDR lets you have both at once... you just take overexposed and underexposed images, plus one in the middle (using a tripod so the camera doesn't move between photos), and combine them into one image.

This technique creates a sort of surreal look, as we're not used to seeing photos in which the highlights and the shadows are so evenly balanced. It makes things look as if they have been illustrated. The look is not for everyone, and some prefer a standard single-exposure photograph.

Don't be fooled by the apparent simplicity of the description here... there is much more to these final images than just combining the three originals. First, to make a good final image requires top-notch equipment to shoot the originals. Then, there are several different software programs used in order to get the detail, contrast, and light levels just right. And of course, lots of practice...