Shutter-Speed is the easiest property for people to grasp because it can have such a dramatic visual impact on a photograph, like that of a waterfall.

Inside of a camera, there is a light-proof curtain (shutter).

The shutter opens to expose the film or digital sensor to light, then it closes to complete the exposure. (Yes, the word exposure  has a few meanings.) 

Now, if something moves, during the duration when the shutter opens until it closes again, it... will... blur. If the Shutter-Speed is set fast enough, our subject will be frozen in place.

In this slideshow example, the Shutter-Speed runs the range from 1/100 of a second (fast/short), down to 0.4 seconds (slow/long). 

Notice how the water, the only thing which is moving, changes in appearance from one photograph to the next.

In photographing waterfalls, I personally find that 1/2 second to 2-seconds (or so) can often deliver a very pleasing photograph. 

Of course, there are other variables to consider, not the least of which is water-volume based on the time of year and whether it's after a heavy rain storm, but I generally find that this is a pretty good starting point.

"Once photography enters your bloodstream, it is like a disease."

Anonymous