[EDIT: The original main question here was "What is Highlight Tone Priority?" It turns out that Highlight Tone Priority, Active D-Lighting, D+, and Adaptive Dynamic Range all mean the same thing, so ...
What does Adaptive Dynamic Range do to a photograph? How does it do its job? How does it affect the results? When should it be used? Nikon calls this "Active D-Lighting"; discussion of other systems ...