'Shutter Speed' is how quickly the cameras shutter opens and closes. The longer the shutter speed, the more light enters the lens. A short shutter speed will freeze things without much motion... this allows you to photograph fast moving objects such as cars or running water. A long shutter speed will capture movement, and the end result of the photograph will look blurred. This is useful when showing movement/motion.
The diagram below shows how shutter speed works.
My Experimentation
This photograph was taken with a short shutter speed... (1/200)
This photograph was taken with a medium shutter speed... (1/15)
This photograph was taken with a long shutter speed... (1/4)