In general HDR is a High Dynamic Range i.e. light range wider then can take and reproduce todays HW (monitors, projectors, camera chips,...). HDR has special meaning when speaking about photos - it means photos with high dynamic range. This photos could be make by exposing several photos of the same scene with different exposure (for example with AE bracketing), and then putting all photos together with some special software or Photoshop plugin:
http://www.hdrsoft.com/index.html - probably the best bet...
http://www.supportingcomputers.net/A...en/Artizen.htm
http://gl.ict.usc.edu/HDRShop/
http://www.debevec.org/Research/HDR/
http://www.cybergrain.com/tech/hdr/
http://www.easyhdr.com/
A lot of HDR photos can be found at FLICKR:
http://www.flickr.com/search/?q=HDR&w=all&s=int