HDR(I) stands for High Dynamic Range (Imaging) and describes ‘… a set of techniques that allow a greater dynamic range of luminance between the lightest and darkest areas of an image than standard digital imaging techniques …’. This may sound weird, but it isn’t. A photo that was taken, doesn’t always picture the scene as the eye has seen it. There will always be a part in a photo that is either too dark or too bright for the camera, never mind how well exposed it is. And this is where HDR kicks in.
HDR is based on a very simple idea: take three or more pictures of the exact same scene (two will do as well), varying from under- to overexposed and merge them on a computer. This way, a dark space on a photo will get enough detail because of the overexposed photo, while a really bright space will get more detail because of the underexposed photo. Merging the photos will therefore not only give you great color, but also detail. I might add though, that the result is a photo that depicts a mixture between substantiality and fiction. Depending on your settings, the result has the tendency to be the latter, with overly bright colors and extreme details.
A really nice read and how-to from Markus Urban is available at abuzeedo.com.
To achieve this, you have quite a few software alternatives to choose from. The one mentioned in the article: Photomatix Pro, if you don’t mind spending a few € or $ that is. I can’t promise great results, but this program is widely used and it even has a Lightroom plugin, so that you can just select the images, export them, create a HDR-image and re-import them directly. Another possibility is Photoshop. If you know your way around, this should serve you well. But there’s also a free program available, which is called Qtpfsgui and is waiting to be downloaded from here. One great thing about this software besides being free, it’s running on all platforms (Mac OS X, Linux, Windows).
Flickr offers various groups, where people may help you on how to get the perfect HDR-image done. This is an example from me:
Photos as taken:
Underexposed:

‘Normal’

Overexposed:

HDR-image result:

This and more photos from me are available at www.flickr.com/photos/blogable/.