Light rays travel in straight lines from the light source. Studying a shadow's shape is an important proof of this characteristic of light. The light source here is supposed to be a « point source ».
The shadow of an object on a screen is the area of the screen which is not reached by rays from a light source.
A common misconception is that a light source creates a shadow behind an object. In fact, the source adds light to areas all around the shadow, and it is the resulting contrast that makes the shadow appear.
The form of the shadow depends on the relative positions of the source, the object and the screen.