Neutral Density (ND)
FiltersND filters are filters that allow you to reduce the amount of light entering the camera. These have to be used when shooting at long shutter speeds in the daytime. Once you have stopped down your aperture all the way and set your ISO to 100, you will still be left with a shutter speed to fast to smooth a waterfall. We will get more into ND filters at a later time.
A flash, or strobe, is a device that bursts light. Most cameras have a "hot shoe" to mount a flash on. When mounted to the hot shoe, the camera can control the flash to create the right amount of light based on the settings.