HDR or High Dynamic Range photography is using at least three digital images to be combined into one image. The special thing about HDR photography is that one of the images is exposed correctly, one is under-exposed and one is over-exposed, both by usually two F-stops bringing out details in the shadows and highlight's, thus High Dynamic Range.
Depending on the way you tweak the images they may look strange at first as they are impossible images when thinking and comparing to traditional photography. At times the HDR effect is more sublte. Lately I am experimenting with underwater and IR or Infrared HDR photography with good results.
I use the Photomatix 4.2.4 software and most of the time the images go through Adobe Photoshop Lightroom5 as well as Adobe Elements 9 post capture software. Almost all of my black & white conversions have been done in Nik Silver Effex Pro 2.