Shady Goings On

B: Belaying

Generally (when using a rope for protection) rock climbers will climb in pairs: the lead and the belayer. They are linked by a rope tied onto the lead climbers harness and passed through a special deviceon the belayers harness. As the lead climber progresses on the route he/she places anchors into the rockface and clips the rope in, the belayer then lets out only as much rope as is necessary.
By doings so, the amount of slack rope is kept to a minimum and the drop is minimised in the event of a fall. The belayers job is to lock the rope off so that no more passes through the device and the lead climber remains safe. Should the belayer fail to hold the lead climber then the fall could be significant and either party may be injured.

The belayer uses specialist equipment to hold the rope, there are many types but they all have some things in common: depending on their use the friction on the rope will either be minimal (e.g. during climbing) or substantial (e.g. when not climbing and definitley during a fall). Switching between the two uses is very easy and is usually as simple as pulling the ‘dead’ end (i.e. the opposite end to the climber) of the rope in a downward direction to increase the friction. The equipment attaches to the belayers harness using a carabiner with the rope passing through both it and the carabiner.

When the climber reaches the top (or during a multi-pitch route, any suitable position) the roles switch. The lead climber (now called the belayer) anchors themselves safely and then belays the belayer (now called the second) up the route from above. Normally the route would be complete when the second reaches the belayer, but for longer climbs (multi-pitch) the second (now called the leader) will continue up and the whole process repeats itself.

Check out the rest of the Rock Climbing A to Z Series

External Links:
Belaying (Wikipedia)
How to Belay when Rock Climbing (eHow)
Belay Devices (Rock+Run.com)