Forced Perspective
Forced perspective is a technique that employs optical illusion to make an object appear farther, closer, larger or smaller than it actually is. It is used primarily in photography, filmmaking and architecture. It manipulates human visual perception through the use of scaled objects and the correlation between them and the vantage point of the spectator or camera.
For a full explanation, read the Wikipedia page. Below are two videos showing forced perspective in the Lord of the Rings films and a music video that uses forced perspective in a very creative way.