A cable release in its most basic form is a shutter button on the end of a cable. While it usually connects to your camera via a cable, wireless releases are also available. The main advantage of a cable release is for long exposure photos, which when used in conjunction with a tripod, reduces the amount of camera shake, reducing the amount of blur in the photo.
Cable releases are also very handy when using the bulb function on your camera. The cable release usually incorporates a button lock which allows you to keep the shutter button depressed, meaning that you don’t need to keep your finger on the shutter button for long periods of time!
Some cable releases also have digital controls, allowing intervals between shots, as well as number of shots taken to be counted and controlled.