The tilt-shift lens allows you to shift and/or tilt a lens on an SLR somewhat like you can on a large-format camera. You can SHIFT the lens to correct perspective distortion, and you can TILT the lens to allow for deeper or more shallow focus.
For years the tilt-shift lens was an obscure item that only serious architectural photographers would use — to straighten up the perspective of building images. But the tilt-shift has become strangely popular to use for an oddity: certain scenes will look like miniature toy sets if shot just right with a tilt-shift lens. It is an uncanny effect, and you have to really look closely to believe it is NOT a toy set. Problem is, once you are over the surprise, what is the point?
Then the question comes up: can you do most of what the tilt-shift lens does in computer later on? Can't you adjust perspective? Can't you produce the oddly short depth of field look in a scene shot at f22 with a wide-angle lens? Of course you can. Which I suspect means that real tilt-shift lenses will go back to being used by only a few serious architectural photographers.
Bottom Line: The tilt-shift lens allows the SLR to have some of the abilities of the large-format camera to adjust lens position so as to adjust perspective and focus. Some of these abilities can be produced later in computer from normal lenses. Tilt-shift lenses can be coaxed into making certain scenes look strangely like miniature toy setups.