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HDR stands for High Dynamic Range, and refers to images which are typically 32 bits per channel, vastly more than can be displayed or reproduced by any current technology. Although HDR sensors are being developed, HDR images are generally created by combining a series of images taken at different exposures, using software such as Photomatix or Hydra. Although the image itself cannot be properly displayed, tonemapping can be used to represent an image which has an apparent dynamic range vastly greater than that which the display or paper is capable of showing.
A pseudo-HDR image is an image which has been created by combining different developments of the same Raw file.