Originally Posted by alcheng

It doesn't cause the vignette, but a tilt-shift lens will change the depth-of-field of the image, it can also "shift" the position of the subject in the image, imaging you bring two distanced subject to same position in a composition, that's how the call it "shift".

When master it well, it can make an actual scene into a miniature scene
It's a special effect lens, pricy and difficult to master....

Here's a wiki ==> Tilt-shift_photography





disclaimer: I didn't take those pics...