The shutter determines how long the film is exposed to light or to the image coming from the lens. Shutters are of two types: the one located just behind the lens, called the leaf shutter; the second type is located in front of the film plane, it's called the focal plane shutter. The shutter consists of two metal sheets or "curtains" which remain shut or closed when the camera is not in use. But when the shutter release button is depressed, one of these curtains slide open to allow the image from the lens to hit the film. After a brief moment, the second metal sheet of curtain will slide in to close the opening. The interval between the opening and closing depends on the speed we selected using the shutter speed knob.
Article Source: http://EzineArticles.com/2519734