Out of the three parts of the triangle, shutter speed is perhaps the one that needs the least explanation. It may also be the part that has the greatest impact on exposure.
Shutter speed is the length of time the shutter remains open to expose the sensor to light going through the lens. It is also what dictates how a subject is captured, especially when it is in motion. At fast shutter speeds, the camera would be able to freeze the subject and a sharp picture. Lower speeds, in turn, leads to moving subjects in different positions between the shutter’s point of opening and closing. This causes what is called motion blur. This is also exacerbated by camera shake, or the inadvertent movement of the camera itself while the shutter actuates.
Why bother shooting at different speeds? To answer that, we need to go back to the fact that you need a fair amount of light in order to achieve good exposure and capture more details. If you go too fast, you can end up with a dark and/or noisy picture. On the flipside, overexposure leads to an overly bright, and in severe cases, white washed picture.
The picture below is a good illustration of the impact of shutter speed in an exposure.
This was shot at a sixth of a second. It was still relatively fast enough to freeze the awaiting passengers in the background. The same goes for the upper body of the lady at the right. However, her legs were moving quickly enough to blur just a little bit on camera. And as can be seen clearly, the the fast moving train is one big blur.
Besides being of help in managing exposure, manipulating shutter speed is a powerful tool in depicting motion, which may play a big part in the story you’re trying to tell in your picture.
I don’t believe in any sort of rules regarding how to set your shutter speed. I always let the environment and my imagination dictate it. So, my advice when learning… discover how fast your camera would shoot. If you are not satisfied with the results, tweak from there to realize whatever you are envisioning for the picture.