If you know what shutter speed does, then you know how to control one of the great creative aspects of photography. Shutter speed is not actually a speed at all—it is a duration of time. In fact, shutter speed refers to the duration of time that the shutter is open, and during which light hits the recording medium (sensor or film). Generations of photography students would not have been confused if the term “shutter speed” had never been used!
With this image of trees along the River Seine in Paris, after much experimentation I found I got the ideal partial blur at 3/10 of a second shutter speed, er, duration of the shutter being open—assuming a slow and steady up and down vertical motion of the camera.