A fast shutter speed will freeze motion.
A slow shutter speed will blur motion.
If movement is the most important part of your photograph select the appropriate shutter speed first then match it to the correct aperture.
The examples detailed on this page are really just a rough guide. The key is to experiment and build up experience over time. You should find the suggested exposure times a good place to start.
Fast moving objects (birds in flight, running animals, moving vehicles, sports):
set a fast shutter speed, at least 1/500". The faster your subject is moving the faster your shutter needs to be to capture the subject sharply. For birds in flight you may need at least 1/2000.
Blurring water movement (waterfalls, rivers, waves):
to give running water a creamy, smooth appearance set a slower shutter speed, 1/2" or more. The longer the shutter is open the more smooth the water will appear to be, the slower the water is moving the longer the exposure time required.
To photograph a reflection on a calm lake 1/2" should be adequate.
To make the ocean appear calm and smooth you will need an exposure time of at least 30 seconds. Even with a small aperture and low ISO setting, exposures of this length may not be possible without using a Neutral Density (ND) filter.
photographs of stationary things, like buildings or mountains, can really be enhanced by blurring the movement of clouds. Obviously you need moving clouds and, unless the clouds are moving quickly, you may need a shutter speed of several minutes.
Panning is tracking a moving object with a moderately slow shutter. The aim is to give the appearance that the subject is stationary while other objects have motion blur. The faster your subject is moving, the faster your shutter speed needs to be. A racing car for example could be captured at 1/125 and still produce motion blur while a slow moving tram or cyclist could be shot as slow as 1/15.
Moving points of light
such as fireworks, light trails, and star trails. A slow shutter is needed to make points of light appear as lines. As with previous examples, the slower the point of light is moving the slower the shutter speed should be set.
Fireworks: between two and five seconds exposure time should work well. Longer exposure times will increase the chance of capturing more explosions in the frame which can either look great or too busy.
Light trails: lights from vehicles that move quickly, like trains or cars on the freeway, will start to show light trails relatively quickly. The longer your exposure time, the longer and more continuous the trails will appear. You can time how many seconds it takes for a vehicle to enter your frame and disappear from it. If you set the shutter to a longer time then the light trails will be continuous throughout the whole frame.
Star trails: The apparent movement of the stars (it's the earth that is actually rotating) is quite slow so a very long exposure time is required to capture their light trails. Around three to four hours exposure time should produce a beautiful image. A note of warning for digital users, a four hour exposure may well cause your sensor to overheat. You'll need to use a timer to record a series of shorter exposures then stack them using software.