Motion blur is the apparent streaking of rapidly moving objects in a still image. It results when the image being recorded changes during the recording of a single frame, either due to:
1- Moving subject
2- Shutter speed (or long exposure).
Shutter speed controls the amount of motion blur in an image regardless of how fast the subject is moving. The slower the shutter speed, the more blur will be introduced to your image because more time is passing as the image is taken. Here’s an example of moving water taken with an iPhone 4S. The first shot has a high shutter speed (fast exposure) of 1/250 and the second shot has a slow shutter speed (long exposure) of 5 seconds.
Shutter speed: 1/250 sec
shutter speed of 5 secs