Tilt-shift is an old photography technique. It consists in tilting the camera's objective or turning it just the other way round. The images you get look like miniatures of the real scene. It's like looking through the wrong side of the binoculars, the depth of field shrinks cosiderably, so only one part of the picture looks focused, all the rest is blurred.
There's also a way to fake this effect using Photoshop when you don't have a reflex camera to play around with the objective. Once I used this technique, when I didn't even know about all this. My purpose was completely different, I just wanted to emphasize one part of the photograph.
I blurred the left part of this picture, reducing the depth of field, and the result was that the people looked like miniatures.
Months later I read about the reason why it happens. It's interesting how our mind interprets this kind of little details.
This, this and this pictures are much better examples of the technique.
There's also a tutorial on how to fake this effect in Photoshop.