Shutter

A movable physical barrier inside a camera or a lens which normally does not let light pass. However, when triggered, the shutter opens for a usually brief and precisely calibrated period of time before springing shut again.
There are two fundamental ways in which a photographer controls the exposure of an image 
i) Controlling the amount of light entering the lens and controlling the duration of the exposure. The lens aperture determines the former. 
ii) The time duration of the open shutter, the shutter speed, determines the latter.

Shutters are generally of two types - focal plane shutters and leaf shutters. They can be mechanical or electro-mechanical in nature. In rare specialized applications they can also be purely electronic - for example, it’s possible to build a liquid-crystal shutter - though such shutters are not seen on ordinary cameras. And some digital cameras which use CCDs control the time duration electrically within the chip.

Of course simple home-made pinhole cameras can have a shutter as simple as a flap of black tape or a sliding metal disc - precise shutters are usually required with lens-based cameras, where exposure times are normally shorter and timing more critical.