The human eye can only see so much information per second, anything faster than this is only seen as a blur. Stating that conversely, the image that the eye sees is held for a period of time.
Film and television use this to their advantage. Frame rates above a certain speed aren't noticeable, so to conserve radio spectrum bandwidth they don't film faster than that rate. Conversely, a frame rate that's too slow, will strobe badly; so the frame rate has to be within a certain range, to be viewable. Also, if the frame rate was too slow, motion would seem very stuttered.
Film uses a projection trick to artificially increase the frame rate: The projector strobes each frame more than once), and television uses interlaced video (see interlaced video, for more information).
The range of frequencies that's (usually) applicable to the human eye, are approximately thus:
16 Hz, so that objects appear to be in real motion.
24 Hz, for a more natural (less stuttered) appearance of motion.
48 Hz, for almost no perception of flicker.
By the way, other animals, and creatures, usually have different persistances of vision than humans.