Cable Releases or remote shutter releases are mechanisms that, when attached to the camera, allow photographers to operate their cameras from a distance. They generally consist of a length of cable or lead with a plug on one end connected to the camera and a switch lock and release button on the other.
After investing in one, you'll wonder what you ever did without it!
They perform two main tasks:
1) In low light situations, where the shutter is required to be kept open for long exposures, for example, photographing star trails where the shutter may need to be open for two or three hours and longer.
2) To assist in close-up photography or when the operator is concerned about possible camera movement as a direct result of manually operating the shutter.
The switch lock/button end of the lead is held by the photographer and once the photo is composed the button is then pressed to release the shutter. If the shutter is required to be kept open (as with the star trails) the photographer presses the button and locks it down with the switch lock and then after a predetermined time the switch lock is released and the shutter closes.