"Negative space, in art, is the space around and between the subject(s) of an image. Negative space may be most evident when the space around a subject, and not the subject itself, forms an interesting or artistically relevant shape, and such space is occasionally used to artistic effect as the "real" subject of an image."
This is a bit more high brow than my usual take on negative space, or perhaps more technical. A lot of my photography is pretty subject oriented, especially wildlife shots. I may show it off centered in its habitat, but the subject usually fills as much of the shot as my paltry 300mm lens can make it. That being said, there are times when the subject just really doesn't make sense out of the context of its greater surroundings. See? Now they have me doing it...in less technical terms, and more true to what's going through my head, I'd simply say, "sometimes you have to see the whole picture to appreciate a piece of it.".
So here's a few of my favorite shots that make a little use of negative space.
From top: Top of the World (Houston Skyline), Half Dome Sunrise, Reeds in Still Water, Hula Abstract, Evolution, Flatlands