HDR. High Dynamic Range
It is post processing a series or one photo in an editing program, combining them and then adjusting the setting in the program to achieve your desired effect.
This method of editing is useful when there are extreme light levels, (high dynamic range) when there is very dark and very light in the same scene, the camera has a very hard time capturing this, unlike our eyes, so there is the option to take various shots to expose for each different light and combine them later. You can get much more detail in photography with this technique.
But, when to use this technique? Lets say you are in the living room and outside the light is very bright and inside it is dark, the camera can´t take just one photo of both lights, so you can bracket for both lights and then combine the photos later one. A bright sky, with a dark subject in the foreground is another, although people and moving subjects are not the best for HDR.
You can shoot in bracketing mode or use the RAW option on your camera. Although the best method is to bracket, take three or more photos (depending on the options on your camera, my D90 only allows for three).
Bracketing is sometimes called auto bracketing, exposure bracketing or look for the -2, 0 +2 on the camera menus. The bracketing button on a Nikon looks like this BKT and on a D90 is on the left, very close to where it says D90. If you don´t have a bracketing mode but you do have manual options you can still take various photos with different exposures manually. I recommend aperture value, and then adjust the settings to the -2, 0, +2.
Chose the bracketing mode, -2, 0 +2, if you have the option to take 5 photos then do so but 3 are normally enough.
Shoot in Raw if you can as this contains more light information, but jpg is also fine.
The most important part is to use a tripod, the programs to a fairly good job with aligning the photos but a tripod is best.
The best program for HDR is Photomatix. I won´t go into a full tutorial for this just follow this link.
(Trey Radcliff is considered one of the masters on HDR, some of his photo are amazing and he has also had the chance to photograph some amazing places and sights. )
If you only take one RAW file and then want to post process it, open it in Photomatix and use tone mapped option. But whenever you can, take 3 or more photos.
Photoshop also has an HDR merge. Follow these instructions. Open the files in PS. Click on File-Automate-Merge to HDR. A window will open. Choose Add open Files-OK.
Wait for the program to do its job, then it will show you a preview of the finished job. An on the right a bunch of options-play around with these until you are happy with the result. Click OK. The finished result will open in PS. Here you can also play around as much as you like. Then save, remember to use a different name for the file and also to remember where you save it.