Shutter Speed, simply put, controls the speed at which the camera's lens opens and closes. It works in conjunction with film speed (a.k.a. ISO) and aperture (lens opening size) to correctly expose an image. By using the Shutter speed priority setting on your camera, you are able to control motion and capture your moving subject effectively. Let's look at the various ways to do this.

Freezing Action
You're at the Indy 500 watching your favourite racer approach the finish line, and you want to capture the car, in all its detail, and the look on the face of the driver, just before he crosses the finish line! Capturing such a shot on your camera is an example of freezing action. Compositions that showcase frozen action allow us to study a fleeting moment at our own pace, long after the moment has passed.

To freeze action with your camera, you must set the shutter speed to be very quick. If your ISO is low, say 50 or 100, set your shutter speed to 1/500th of a second, 1/1000th of a second, or faster. Then let your camera recommend the apertureit will most likely force your background to be out-of-focus with a large aperture. If your ISO is high, say 800, then you'll be able to set your shutter speed to 1/500th of a second without losing quite so much detail in the foreground and background (i.e. you'll be able to use a smaller aperture to get the right exposure). Shooting at such quick speeds allows you to get away with not using a tripod, since any shaking of the camera you do accidentally will not affect such a short exposure. So pick a sunny day, and go for it!

Implying Motion
The classic example of implying motion is those ethereal shots of rapids or waterfalls where the water looks like silk as it flows over the rocks, meanwhile the plants on either side are in sharp focus as the water passes by. Another example is traffic at dusk while the buildings at the intersection stand quietly in sharp focus. The subject is blurry while the background is sharpthis is what implies the motion.

The trick to capturing implied motion is to use a slow shutter speed. Speeds of ? second should suffice to snap waterfalls or rapids, but be careful to decrease your ISO, or shoot at night, and don't forget your tripod.

Panning
Your daughter is riding her tricycle in a circle around you. You point your lens at her and pivot at the same pace that she circles you. Snap! This is an example of panning, where we create anxious and hurried moments by putting the moving subject into sharp focus while blurring the background. The best panning shots are captured when the subject is looking at you, and the eyes are in focus.

To successfully capture a panning shot, consider these three rules:
1) Make sure you move in parallel to your subject.
2) Choose the right shutter speed: somewhere between 1/60 second (for quick subjects), 1/30 second, or even 1/15 second (for a slow child, say)
3) Do NOT use a tripodyou must be free to move

Now you know the secrets to controlling motion on film. Get out there and try your hand at freezing motion, implying motion, and panning!