There are three main styles of belay device: tube style, autoblocking, and figure 8. Each have their own unique advantages and disadvantages. The most basic and widespread are the tube style belay devices such as the Black Diamond ATC or Petzl VERSO. The next style is the autoblocking device—these are predominantly used in gym and sport climbing. The final style is the figure 8, these are used primarily for rappelling and only used for climbing in a pinch.
Tube style belay devices are everywhere, if you ask a climber what belay device they use—the overwhelming majority will say a tube style device. Within tube style devices, there are two main branches—the basic tube style and the tube style/guide plate hybrid. The guide plate style does everything the basic can do, with the added benefit of assisted breaking while belaying a follower from the top of the pitch. A basic tube style device is plenty for a beginner climber, but the guide plate style lessens the need for an upgrade in the future. All tube style belay devices can double as a double rope rappel device—meaning you only need to take one device up the wall with you. The biggest drawback to a tube style device is that it requires more force to arrest a fall. They are also slightly more difficult to learn.
Autoblocking belay devices are the easiest to use and learn. There are two styles under the broader autoblocking label. The first, and most common, is the active autoblocking device. The most well known of this style is the Petzl Grigri 2. These devices have a camming mechanism inside that rotates and pinches the rope, creating enough friction to hold a fall. Unfortunately these devices are not capable of double rope rappels—meaning you will need to bring along a second device if a rappel might be needed. If you are climbing in a gym or doing sport routes where you will be lowered, this is not a concern. The second style of autoblockers are the passive devices such as the Edelrid Mega Jul. These devices work by rotating the angle of the device, increasing friction on the rope and holding the climber. Some of these devices are capable of a double rope rappel, others are not. You will need to look at the individual device you are considering.
The final style is the figure 8 belay device. Figure 8 devices are rarely used to belay, they generate less friction than the other devices, making belays more difficult. They also twist the rope, leading to kinks. They do, however, give the smoothest rappel of any device. They are a good secondary option if you opt for a belay device that doesn’t allow for a double rope rappel.
All these devices have advantages and disadvantages. It is much easier to move from a tube style device to an autoblocker—meaning you will not need to relearn technique. Tube style devices also require more concentration than the autoblockers—which will train you to pay constant attention. Autoblockers sometimes allow you to become complacent—and complacent belayers cause injuries. Take a course with a guide, like at Denver Climbing Company, to safely learn how to utilize these devices.