Good old Wikipedia helped for this explanation:
"Tilt-shift photography" refers to the use of camera movements on small- and medium-format cameras, and sometimes specifically refers to the use of tilt for selective focus, often for simulating a miniature scene. Sometimes the term is used when the shallow depth of field is simulated with digital postprocessing; the name may derive from the tilt-shift lens normally required when the effect is produced optically.
"Tilt-shift" actually encompasses two different types of movements: rotation of the lens plane relative to the image plane, called tilt, and movement of the lens parallel to the image plane, called shift. Tilt is used to control the orientation of the plane of focus (PoF), and hence the part of an image that appears sharp; it makes use of the Scheimpflug principle. Shift is used to adjust the position of the subject in the image area without moving the camera back; this is often helpful in avoiding the convergence of parallel lines, as when photographing tall buildings.
Enjoy the pictures!