It stands for High Dynamic Range.

It’s a feature in Photoshop CS2 or Photomatix or FDRTools.

It’s basically a way to ‘digitally develop’ your photos using tonal information contained in multiple exposures of the same shot.

Say you exposure bracket the same shot. You can merge all three files into one 32 bit file and use the data contained in the images to create one master output preserving accurate details from all exposures.

FROM WIKIPEDIA:

In computer graphics and photography, high dynamic range imaging (HDRI) is a set of techniques that allow a far greater dynamic range of exposures (i.e. a large difference between light and dark areas) than normal digital imaging techniques. The intention of HDRI is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to the deepest shadows.

HDRI was originally developed for use with purely computer-generated images. Later, methods were developed to produce a HDR image from a set of photos taken with a range of exposures. With the rising popularity of digital cameras and easy to use desktop software, the term “HDR” is now popularly used[1] to refer to the process of tone mapping together bracketed exposures of normal digital images, giving the end result a high, often exaggerated dynamic range; however, in this case neither the input nor the output qualify as “true” HDRI.

More information here:

http://en.wikipedia.org/wiki/High_dynamic_range_imaging

www.luminous-landscape.com/tutorials/hdr.shtml

Recovering HDR from images

Rendering with Natural Light

www.astropix.com/HTML/J_DIGIT/PS_HDR.HTM

Diffs btw HDR (tone mapping) and exposure blending.

www.cybergrain.com/tech/hdr/

tutorial for handling moving objects in FDRTools

have fun!