For many years a back or shoulder belay was all climbers protected their leader or second with - until the late 1960s when Franz Sticht invented his eponymous Sticht Plate, a revolution in mountaineering safety. Nowadays a belay device is considered essential for climbing and mountaineering but the array on the market can be bewildering. Here are a few notes to help.
If you have any intentions of climbing outside on 'trad' climbs, a belay plate with two slots is recommended as you will probably want to use double ropes at some time, and even if you don't, you will almost certainly have to abseil down double ropes. The safest designs are those that have grooves of some sort that grip the rope such as the BD ATC-XP, Petzl Verso etc, as they have greater braking effect than other plates. And, as lead ropes get ever thinner, this extra braking becomes more important.
Most Belay Devices can also safely be used to abseil with in most situations and few climbers these days would bother carrying a separate Figure of Eight for abseiling (though be aware that they heat up more quickly than a large figure of eight descender). Use of a Prusik Loop or a Shunt to safeguard yourself while descending is highly recommended.
For sport climbing there is a good selection of belay plates designed to make holding falls and holding or lowering off a dangling climber safer, which they do by greatly increasing the friction. Some have moving parts and others manage almost as well without. They almost all only work on single ropes so are not much use for trad climbing, and one would need to carry a separate abseil device on multipitch routes. However, for anyone who goes sport climbing regularly, it is well worth having one sport belay device per team.
Please check out our section on Abseil Knots for some important and potentially life saving information.