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, from the time the shutter opens, until the time it closes again, it... will... blur. If the Shutter-Speed is set fast enough though, it will freeze our subject in motion.

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

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

In photographing waterfalls, I find that 1/2 second to 2-seconds (or so) can often deliver a very pleasing photograph. Of course, there are a ton of other variables to consider, not the least of which is water volume based on the time of year and whether it's after a rain storm or not, but I find that this generally is a pretty good starting point.

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

Anonymous