Figure 8 (belay device)
The figure 8 belay device is a piece of metal (usually an aluminum alloy) in the shape of an 8 with one large end and one small end.

Usage

This device utilities a large surface area in contact with a climbing rope to provide sufficient friction along with proper technique  to be used as a belay device or for rappelling. A figure 8 (sometimes just referred to as an 8) is used in conjunction with a climbing harness and locking carabiner to keep a climber safe and control their descent or one's own descent (when rappelling)

Pros
Figure 8s offer a very smooth controlled descent when rappelling and lowering climbers. They can be used with nearly any diameter climbing rope and don't get as hot as other friction devices because of their ability to dissipate heat efficiently. The figure eight can also be used with a doubled rope, which is also possible with a rack but not a bobbin.

Cons
The figure eight cannot be removed from the rope without detaching the device from the harness, and so could be accidentally dropped, particularly when tackling a rebelay. They also twist the rope which is a particular problem where a rebelay prevents the rope untwisting. They do not provide a convenient means to temporarily lock the descent at a rebelay.