The passage that rays of light take going through the camera lens and onto the sensor (or film) must first pass the shutter – which for the most part remains closed.
But it does open up! Usually for just fractions of a second – even for a thousandth of a second! Other times longer.
A lot of how you set your shutter speed would be based on achieving a proper exposure (Read the Water In A Bucket article. In this case Shutter Speed is represented by the faucet)
There is however one main consideration when you change your shutter speed, and that is motion blur; those fuzzy streaks that can ruin a photo just as easily as it can enhance it.
Want to make something appear to be moving?
Suggesting motion in still images can be impactful at times, however too much motion blur, or any motion blur at all can be the clear cut deal breaker between a good photo and a bad one.
If you experience motion blur when you do not want any, it’s usually an indicator that your shutter speed is too slow, and that you need to make it faster, and balance your exposure out by adjusting your other settings.
The opposite holds true, if you feel, as the creator of the image, that adding motion blur would in fact add value to the final result, then perhaps you should consider slowing the shutter speed down until a the desired effect is achieved.
Experience is a great teacher. The more you shoot, the more opportunities you will have to exercise what you know and, in turn, the better you will be.