Taking photographs with long exposure settings requires a steady camera, which is why the tripod is an integral part of any setup. By that same token, even a camera on a tripod will be rattled by a photographer depressing the shutter button. Even though using the tripod is far more stable than shooting by hand, you can do better. Enter the cable release, or shutter release, a camera accessory that provides an external shutter button for triggering shots.
Cable releases are so named for the cable that plugs into the camera and adds several feet of separation between the shutter button on one end, and the camera on the other. Of course, these days some shutter releases do without the cable and work wirelessly. That's helpful if you're worried about tripping over the cable.
While both wired and wireless cable releases are good for reducing camera shake, many of them offer another useful function: locking the shutter open for an extended period of time. A long exposure shot of the stars, for example, could take hours. Shutter releases make that possible, and they're cheap.