High Dynamic Range (HDR) images are created by combining information from multiple exposures of the same scene, usually, but not always, changing only the shutter speed over a range of stops. The information in these exposures can be blended to optimize tonal balance and detail throughout the entire image.
Film photographers, especially landscape photographers, have long utilized neutral density (ND) filters to help cameras manage the much broader range of light perceived by the human eye. More recently, digital photographers have sandwiched multiple exposures - or multiple processes of single RAW exposures - using masking and opacity to achieve the desired effect.
Now, software tools such as Photoshop CS2, image-makers can use complex floating point 32 bit (per channel) math to accomplish this, and more. This series represents some subtle, and not-so-subtle, uses of this technique.