Shutterspeed

   The shutterspeed determines how long the film or sensor is exposed to light. Normally this is achieved by a mechanical shutter between the lens and the film or sensor which opens and closes for a time period determined by the shutterspeed. For instance, a shutter speed of 1/125s will expose the sensor for 1/125th of a second. Electronic shutters act in a similar way by switching on the light sensitive photodiodes of the sensor for as long as is required by the shutterspeed. Some digital cameras feature both electronic and mechanical shutters. 

   Shutterspeeds are expressed in fractions of seconds, typically as (approximate) multiples of 1/2, so that each higher shutterspeed halves the exposure by halving the exposure time: 1/2s, 1/4s, 1/8s, 1/15s, 1/30s, 1/60s, 1/125s, 1/250s, 1/500s, 1/1000s, 1/2000s, 1/4000s, 1/8000s, etc. Long exposure shutterspeeds are expressed in seconds, e.g. 8s, 4s, 2s, 1s.

   The optimal shutterspeed depends on the situation. A useful rule of thumb is to shoot with a shutterspeed above 1/(focal length) to avoid blurring due to camera shake. Below that speed a tripod or image stabilization is needed. If you want to "freeze" action, e.g. in sports photography, you will typically need shutterspeeds of 1/250s or more. But not all action shots need high shutterspeeds. For instance, keeping a moving car in the center of the viewfinder by panning your camera at the same speed of the car allows for lower shutterspeeds and has the benefit of creating a background with a motion blur.