The Shutter of a Camera

The Shutter of a camera is what allows light traveling through the lens hit the film/sensor to take a picture. To get a well exposed picture a photographer uses a combination of shutter speed, F/stops, and ISO settings. These are different numbers that control the amount, timing, and amount of absorption of light during the capture. The ISO considers the absorption of light on to the film or sensor while the F/stop (or Aperture) deals with the amount of light hitting the shutter. The Shutter speed is very important because it controls how long the light is allowed to expose the sensor or paper. 
Just like when we made pinhole cameras and we had to count while holding open the piece of tape blocking the lens the shutter concisely controls the time that the lens remains open during exposure. The shutter can remain open anywhere from 1/8000 of a second to hours at a time depending on the need for a well exposed picture. A well exposed picture means the lighting looks correct as you see with your eye and things are sharp in the frame. 

You can utilize the shutter speed to play with the motion in a photograph. Slower shutters will capture movement in action (Things will start to blur if they move during exposure) or you can freeze something mid-action (When you can sharply and clearly see an object that was in motion during exposure).

To help really understand how much time an exposure is a photographer will use decimals to clearly see how long or short the shutter is set at. 
This really helps studio photographers who need the perfect shot understanding this concept helps them spend less time getting one shot. This is because not only does their camera have a shutter speed their lighting equipment has to be synced up with the camera. This allows the photographer to use different lighting settings for one exposure. For example if the photographer needs more light in the background compared to the foreground he can adjust the timing of the lights to go off at certain times during the exposure. He needs to understand his timing to do that. And to understand his timing he needs to understand decimals because shutter speeds are mainly decimals.