Tilt/Shift lenses basically controlls the vision by the orientation of the lens with respect to the film or sensor . Tilt helps in achieving a shallow depth of field while shifting have wider field of vision .
In Normal lens image sensors, lens and objects are parallel to each other and hence the objects which are in focus are at the same distance from the camera . So when the lens is tilted w.r.t sensor the point of focus is at an angle to the sensor and objects at different distances from the viewfinder can have a sharp focus. This cannot be simply achieved jus by tilting the camera or your body :) and offcourse cannot be achieved using a large aperture . Looking down at an angle to the object can actually have a tilt/shift miniature affect . Having a shallow depth of field in the photos is very much important if you want to give the illusion of focusing the Camera.
These lens are usually used for taking photographs of the building like in architectural photography which when taken by the ordinary lenses gives you only the bottom portion of the building . Tilting the lens upwards gives u a vertical perspective and shifting gives you the entire building by changing the line of sight.
After googling I found out this website which was very usefull in creating the tilt/shift miniature affect on ordinary photographs . Where you can select a narror or wide area of fucus which will be sharply focused and the rest of the area will be out of focus with the specified amount of defocus strength and gradient.
