What is Shutter Speed ?

It's the amount of time the shutter opens to let light into your camera.

Rated by seconds:

• 1/1000 s = high / fast shutter speed.
• 3 s = low / slow shutter speed.

Related: Aperture, ISO.

• Fast shutter (1/100 s) = requires bigger aperture, small DOF, may require high ISO.
• Slow shutter (3s) = smaller aperture, large DOF, allows low ISO.
  
  

  Motion blur	wikipedia.org
  High shutter speed, freezes action.	Low shutter speed, causes movement blur.
  Night Photography	photoxels.com
  Fast shutter, darker photo, less light.	Slow shutter, brighter photo, more light.

In short:

• Fast shutter speed to freeze action, slow shutter speed for motion blur.
• Slower shutter speed for more light.

 

References:

• http://en.wikipedia.org/wiki/Shutter_speed
• http://digital-photography-school.com/shutter-speed
• http://www.mir.com.my/rb/photography/fototech/apershutter/shutter.htm