HDR Imaging (see Wikipedia Article) allows the production of photographs which incorporate a much greater dynamic range between light and dark than can typically be produced from a single shot taken by a digital camera.
These images can be a more accurate reflection of what the human eye sees and, (as is the case with this shot) can incorporate an element of artistic license to produce more dramatic results. Here the resulting picture was much closer to what my eye was seeing in West Hampstead this evening than any single image off the camera could produce.
3 shots were taken at ISO 200, f/7.1 +/- 2 Stops (1/30, 1/125, 1/8). The images were merged using HDR Efex Pro 2.