Descenders or belaying devices are used in top rope and lead climbing. There a couple of different types: tubular, assisted-braking and figure 8. A tubular descender is a two-part system, the tubular descender and a locking carabiner, it's a simple and safe system to let you make soft catches and lower smoothly. There are also double tubular descenders too for double rope climbing and more advanced belay techniques. Assisted braking devices like the popular GriGri have an internal cam which 'grabs' the climbing side of the rope in case of a fall. Other assisted braking devices use a carabiner to 'pinch' the climbing side of the rope and slow it down in case of a fall. Finally, figure 8 devices are manual belay devices that rely on the belayer doing more work to catch falls and belay the lead climber, these are useful for long descent and tough conditions but are increasingly banned on indoor walls.