Thus, we conclude that the shutter speed is affected by the ISO, the aperture, as well as the light condition. For instance, suppose we want to fill a very big water bucket with a narrow-opened faucet in the night. We have to open the water for very long time since the water pressure is low and only the small water can reach the bucket at a time (due to the narrow-opened faucet).
The following picture shows that if we use a big faucet (or a large size of an aperture) with the high pressure pump (or the day light condition), the bucket will be filled very quickly. That means we have to open the faucet (or the shutter) for just a very short period of time. In other words, we use pretty fast shutter speed (usually, 1/125 second).
In the opposite, If we use a small faucet (a small size of an aperture) with the low pressure pump (the low light situation), obviously, we need more time to make the bucket full of water. This implies that we use slow shutter speed. It explains why you usually have to hold the camera longer when you take indoor pictures. Even very little movement or camera shake whilie the shutter is opened can make an image blurred. So, by holding the camera by hand, it's not easy to get a good shot in the low light situation since the camera needs to open the shutter for a long period of time.
One possible way to increase the shutter speed or reduce the time shutter opening time is to increase the ISO sensitivity. By doing that, the light your camera needs is decreased so it takes less time to fill the bucket. However, increasing the ISO has a major side-effect. That's the image will look grainy. Technically, there are a lot of noises in the image.
Another way to reduce the shutter opening time is to turn on the flash. It's like you add more of water pressure or light density. One disadvantage is the picture sometimes doesn't look natural since you use the artificial light from the flash.
(pictures source: go_med @ rpst-digital.org)
ISO, Aperture, and Light Condition part 1/2
Learn about Shutter Speed and why it's so important
Aperture and F-number
More on Shooting Tips