Persistence of vision is when light strikes the retina and the brain retains the image for about a 10th to a 15th of a second after the image has actually been seen, therefore commencing itself to animation by using the illusion of motion when we view images in closely-timed sequence to one another. The eye does not take notice of the fractional skips between images that occur faster than the retention period, therefore the changes either go unnoticed or they appear to be one continuous picture.
source(s):
http://animation.about.com/od/glossaryofterms/g/pervision_def.htm
http://www.vision.org/visionmedia/article.aspx?id=136
