The shutter speed you choose is not simply a way of controlling the overall exposure, as for the amount light is let in. It also has a visible effect on your shots, enabling you to control the appearance of a moving subject.
A fast shutter speed freezes the movement. Slow shutter speeds tend to blur movement, and so can be used for creative effects. The slower the shutter speed, the greater the degree of motion blur in the image you take, normally better shot with tripod, to keep foregrounds and back grounds sharp and in focus.
The shutter speed is the length of time that the shutter is left open. The scale used is easier to understand than the aperture system, because speeds are measured in fractions of a second. However, the numbers are often simplified – so 1/125 sec is shown as 125, 1/15 sec as 15 and so on. Speeds of a second or longer are shown as 1”, 2”, and so on.