A definition of shutter speed is the duration that the camera's shutter will open in order to let the light go inside. After that, films or image sensors start capture the light and turn it into photos. Shutter speed can be considered as a controller for adjusting amount of light for each image.
Shutter speed is literally called exposure time. We measured it in seconds. Normally, an exposure time for shooting in sunlight is 1/125th of a second.
While the camera operates in the Auto mode, it completely controls the speed of shutter. The shutter will be closed when the camera thinks there is enough light going inside. In contrast, if you take a photo with the manual mode, you have to adjust shutter speed yourself. If the speed is too slow (the shutter opens too long), your image will be too bright or white. If the speed is too fast (the shutter opens too short), your image will be too dark or black. Clearly, the shutter speed is the heart of photography. By manually control shutter speed as well as the aperture, you may get fantastic shots you never have with the Auto mode.
Purposes and uses of the shutter speed
Short shutter speed
It is used to freeze fast-moving subjects, for example at sporting events.
Long shutter speed
It is used to intentionally blur a moving subject for artistic effect. Usually, we use long shutter speeds when we try to smooth water flows. For example, we take pictures of waterfalls with very long shutters.
white numbers are the shutter speeds in seconds (source: wikipedia)
Some Canon PowerShot models that can operate in the manual mode are G10, SX1 IS, SX10 IS, SX110 IS, SX200 IS, A590 IS, and SD990 IS.
See also
ISO, Aperture, and Light Condition part 1/2
ISO, Aperture, and Light Condition part 2/2
Aperture and F-number
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