What is HDR photography?  HDR stands for high dynamic range.  Basically the human eye can see the whitest cloud in the sky and the dark ground at the same time--approximately the equivalent of 20 f-stops of a camera.  The digital camera of today can only capture 6 f-stops of light with one image.  If one exposed for the sky and clouds, the rest of the exposure would be underexposed or near black in view.  Conversely, if we exposed for the interior contents, the windows and bright lights would be blown out or overexposed.  

Four images are taken of each view to capture the entire visible range of light-light to dark.  Therefor for the 25 image package, a minimum of 100 photographs are taken and each view is processed with software to capture detail across the dynamic range.  The results are bright, colorful, detailed images closely resembling the view that the eye sees.  Seems like a lot more work, but the results are far better than flash photography.