Shutter speeds go in a particular sequence, where each is twice as fast as the one before (this 2x step is used with ISOs and lens apertures to in order to simplify exposure calculations). Here's the sequence (there are slower and faster speeds too):
1sec, 1/2sec, 1/4sec, 1/8sec, 1/16sec, 1/30sec, 1/60sec, 1/125sec, 1/250sec, 1/500sec, 1/1000sec
The shutter can be inside the lens mechanism (compact cameras) or directly in front of the film/sensor (single lens reflexes and other interchangeable lens cameras like rangefinders). Shutter speed is an important creative control because it can be used to 'freeze' movement or introduce deliberate movement blur.
With moving subjects like this fairground ride, the usual technique is to use a shutter speed fast enough to freeze the movement. In this case, though, a very long shutter speed of 1/6sec was used instead. Using an image-stabilised lens helped keep the background relatively sharp, while the spinning horses have turned into an impressionistic blur.
