Shutter-Speed

0.5 seconds
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 (called a 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.)
If anything moves, from the time the shutter opens until the time it closes again, it... will... blur. If the Shutter-Speed is set fast enough, our subject will be frozen in place. Simple as that.
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 more) can often deliver a very pleasing photograph, but not always.
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.