The shutter is composed of blades, a curtain or some other moveable mechanism that allows light into the camera. (For those of you who watch Stargate SG-1, think of the "iris" on the stargate.) When you press the shutter release button, the shutter opens and closes at a variable speed either controlled by the camera when using either automatic or aperture priorty modes or controlled by the photographer when using manual or shutter priority modes. The shutter speed controls the amount of light allowed into the camera and is measured in steps, generally from 1/8000 to 30 seconds.