Template:Stub As the name implies, a belay device is a piece of equipment to assist you in belaying a partner with a rope. Many different devices are available - be sure you know the particular intricacies of the one you choose!
Basic Belay Devices
The "Air Traffic Controller" is the standard, basic, lightweight belay device.
More Elaborate Belay Devices
This main advantage of this design, or similar ones by different names (ATC Guide, etc.) is the additional attachment point which lets you belay one or two seconds off an anchor from the top of a pitch. Watch the videos below to see how to use ATC guide or Petzl reverso. Especially, see 0:55 and 4:00 respectively in above to see how to lower an ascending climber, while belaying from top. Not really intuitive.
This expensive device is usually considered too heavy for mountaineering, but is static (won't suck rope in under a shock load) and self-locking. Good to use in the climbing gym!
Supertopo.com has a great belay device gear review that covers all the popular types of belay devices. Highly recommended!