HDR stands for High Dynamic Range photography, meaning mixing the same image with different exposure, for example you take the same image but one would be over exposed the other would be under exposed and so on place them together and you would get a very interesting image.
You can get a clearer knowledge of HDR Photography here
Here are some images i found while prowling the internet for HDR photography.
I find these images amazing, they are so out there and beautiful.
all images fhttp://www.google.co.uk/search?tbm=isch&hl=en&source=hp&biw=1366&bih=576&q=hdr+photography&gbv=2&aq=1&aqi=g10&aql=&oq=hdr
