Shutter speed is basically the speed in which the shutter opens and closes. It's simple really. Shutter speed affects the amount of light entering the sensor. Shutter speed, can be used as a powerful tools to exaggerate, or freeze motion. You could freeze a bird in mid-flight, or show a milky waterfall.
If you were to increase your shutter speed, as in a "long shutter speed", you could take a picture of a waterfall, and exaggerate the water falling in a milky white fashion. Not only can you just take a picture of waterfall, but you could take night images. If you increase your shutter speed, more light will enter the sensor, resulting in a "brighter" image. You could light up a city or a bridge at night with a long shutter speed.
Shorter shutter speed, or "very quick shutter speeds" can be used to freeze flying insects flying towards your face. Sadly, it doesn't work in real life, and only works on film, so remember to dodge. The problem with this is, if the lighting isn't bright enough, you may end up with a under exposed picture, and everything could be very dark.
In summary:
Slow Shutter Speed
Pro:
- Blurring effects
- Brighter night scenes
- Increased exposure
Cons:
- Blurry pictures without a tripod
- Over exposed pictures
Fast Shutter Speed
Pro:
- Freezing effect
- Little to no blurring
Con:
- Dark/underexposed pictures
Also, ISO.
ISO? We ain't talking about Tron now. We're talking about how sensitive the camera is towards incoming light.
Generally ISO can make images look brighter, but despite of that, you want to generally keep ISO as low as possible. No matter what camera, the higher ISO, the greater chances, and the great amount of "image noise", or that famed "grain" in all of your dark pictures.
Pro:
- Brightens pictures
Con:
- Decreases picture quality (Image noise)