Consider the shape and size of your shadow when you walk out of your house into early morning sunlight - the shadow is quite long and it widens as the distance away from your feet gets larger. Contrast this with the shape of your shadow in the sunlight at noon - the shadow is much shorter, concentrated closely around your feet. The reason for the difference between the two shadows is the change in position of the light source, the sun.
Shadows are created by a light source illuminating an object that is not transparent; the shadow shows the blockage of light caused by that object (your body in the above examples). As the source of light moves in relation to the object, the geometry of the blocked rays of light changes. This causes the shape of the cast shadow to change. See http://studiochalkboard.evansville.edu/lp-shadow.html for further explanation.