Tilt-Shift photography is basically the use of of camera movements on small and medium size cameras. In many cases, it refers to tilting the lens relative to the image plane and using a large aperture to achieve a very shallow depth of field. The result is a blurring of parts of the image for de-emphasis purposes and the possible effect of reducing huge objects to miniature models.
Sounds complex, but here are some very nice examples that should explain everything.
