As you can see, the TS lens literally tilts and shifts away from the straight line of site. For an Architectural Photographer, it's a must have.
Have you ever taken a picture of a big building while standing relatively near it? The photo you end up with is a building that has a very wide base, but the top is narrow and it just gets skinnier the higher it goes. Well, a Tilt-Shift lens will take care of that. Here is a shot of the Chicago Skyline maxing out the Lens in both directions. It's a good example of what extremes a lens like this can accomplish.
While this example shows you the difference, it also opens your mind to something you might have never even thought was available. Take a look at the following pictures and really note how different they might be with a standard camera lens.