Hi Chris,
D-lighting is an in-camera editing feature found in the latest nikon DSLR like D80, D40, D40x. It's also available in most of the current compacts i think.
It brings out the details in the shadow areas if the shot is slightly underexposed. If i'm not wrong, it's actually the equivalent of photoshop's dodge tool. Just that you have the convenience of applying it immediately after taking the shot.
But there's a catch: It also makes the picture more noisy especially when you're already using a high ISO or the image is already severely underexposed.
D-lighting is also available in the basic program PictureProject that comes bundled with your nikon camera. But the funny thing is, applying D-lightning in astro shots (only) using PictureProject gives really bad results. But it's great when applied through the camera.
Here's an example:

original (ISO 800)

in-camera D-lightning (moderate setting)

PictureProject D-lightning (low setting)
As you can see, the PP gives really bad results even at it's low setting compared to in-camera at its moderate setting. Weird!
Hope this helps!
Cheers