Persistence of Vision

Persistence of vision is a theory that a strip of still images in any order can be given the effect of animation if viewed by the human eye at a certain rate. 
This gives the effect that the image is still there for a split second after it has gone and therefore gives the illusion of animation moving. 
Persistence of vision was not created by any one, instead it was discovered by very early optical illusional toys which showed the theory was correct and accurate.

Here is a youtube clip also explaining Persistence of Vision: http://www.youtube.com/watch?v=iWIXxiMeJIg


Reference: http://animation.about.com/od/glossaryofterms/g/pervision_def.htm
                  http://en.wikipedia.org/wiki/Persistence_of_vision
                  http://www.youtube.com/watch?v=iWIXxiMeJIg