Shutter-Speed  is the easiest property for people to grasp because it can have a dramatic visual impact on a photograph.

Inside of a camera, there is a light-proof curtain (shutter-blade) that opens and closes, exposing the film/sensor to light.

If something moves, from the time the shutter-blade opens until the time it closes again, it will blur.

On the other hand, we can have the Shutter-Speed set fast enough, it will freeze the subject in motion.

In the example shown previously, the Shutter-Speed runs the range from 1/100 of a second (fast exposure), down to 0.4 seconds (a long exposure).

Navigate through and notice how water, the only thing which is moving, changes in appearance from one image to the next.

Take note of the time the shutter-curtain was open. 

In photographing waterfalls, I find that 1/2 of a second to 2 seconds, and beyond often delivers a very pleasing photograph. Of course, it all depends on the light I am shooting in.