This part of composition is really quite simple - but very crucial to taking pictures with artistic appeal. If we all spent a day in art school, the concept of positive and negative space would definitely be one we'd spend some time on. Quite simply, your subject is the positive space - everything else is the negative space.
Okay, great, well what does THAT help us do?
It's just a different way of looking at your frame to help you make creative decisions. Have a hard time deciding if the background is cluttered? Pretend that pretty little face isn't there anymore - now what are you left with? Is it clean? Does it tell a story? Does it take away or add to your subject?
This driveway creates a perfect canvas of negative space for Sydney to take her doll for a walk. It's almost as if it transports you into the imaginary world of a two-year-old going into the great beyond. Even more perfect - the subtle ray of light pointing in the direction beyond.
And how's this for negative space? The dark curtains behind leave nothing to focus on but the sweet sensation of getting valentine candy.
Now lets contrast this to a picture with negative spce that's a little more complicated.
While our rocking princess is as cute as can be - there are elements in this picture competing for her crown. It's not so much that the rocking chair is busy - that's really still part of our positive space since it's part of our focal point. Look at the negative space (everything outside of the rocking chair). It's pierced by harsh lighting differences that subtly distract from our subject. Is this a horrible picture? No. But do we wish the negative space wasn't so dappled? Yes.
Really - understanding positive and negative space is just another way to make sure that your subject is the center of attention. It's the more studied version of Brand Spankin' New's Lose The Clutter lesson. Plus, it makes you sound really smart around all the art people (*wink*)