In the days of film camera, photographers are constantly faced with the dreaded one shot proposition and if his settings are ideal for that particular scenario when the shutter release button is pressed. At least until the print is obtained will he then find out.
The Nikon D90 brought this tradition to automatically capture your subject with a plus and minus "compensation" of your subject, bringing the camera to work thrice as hard to store your shot in multiple (up to 3 shots total) compensated settings - White Balance (K) / ADL / AE / AE and Flash / Flash Only. In other words, when you take that shot, the camera automatically replicates and store up to two additional image that you want compensation introduced.
I personally find ADL (Active D-Lighting) compensation not having a significant impact on the image I want compensation introduced but rather, more in activating the color temperature compensation (White Balance) that I yield rather interesting results. In compensating the WB, we can choose how many "stops" (aka mire) we want the image over and under-balanced.
This "bracketing" feature is now almost always activated on my Nikon D90.