A quick note on why 24 frames per second: During the silent era, most films were played back at around 14-20 frames per second. 24 frames per second became standard once sound was introduced and printed optically along the side of the film. 24 frames a second gave the track sufficient speed for good audio playback without wasting too much film. Also 24 is a nice number divisible by 2, 3, 4, 6, 8 and 12, giving editors lots of different ways to divide up a second.
Source : Filmmaker IQ