In layman terms, HDR (High Dynamic Range) is a post processing method, by which it is possible to get the different lighting levels together, in a single shot (which other wise is not possible with a normal camera).
HDR In Action
Consider the below three shots, that were taken @ different exposure. All three shots had the below EXIF (Only the Exposure was different)
Exposure time 1/60 s + F-number f/2.7 + Max. aperture value f/2.7
|Exposure value 0.00 eV
In this shot with normal exposure, one could see the things inside the room and also, though a bit washed out, could slightly see what is out side
|Exposure value -2.00 eV
This one with a -2 eV captures the outside scene, higher lighting) but the room gets more darker, one could not make out the things on the table
|Exposure value +2.00 eV
This one with a higher eV clearly captures the room, but the higher intense (out side) is totally washed out
The beauty of HDR is that using the above three different exposures, one could build (post process) to arrive at a shot like below.
It is interesting to note that the final (single) shot had clearly captured the coolness in the room (u could even note the table lamp glowing, the reflections on the shiny teak table, the crystal clear glasses, the details on the wall hanging) and the detail in the outside view (the construction happening, the river, mountains, buildings etc)
Click here to look at more of my HDR Samples
The Flickr HDR Group too has a great collection too.
Look out for further posts talking about the Tools required and the Actual Techniques of HDR and other Related Resources