Central Locking Solenoid

What is a Central Locking Solenoid?
The central locking solenoid is effectively just an electrical switch that opens and closes the various locks around the car such as the doors, boot etc. These require power and when they fail can lead to problems getting into the car, especially on newer cars when a manual way of opening the car isn't always straight forward.
What problems can be caused by this part?
How much does it cost to replace the Central Locking Solenoid?
They tend to cost from around £20 per unit.
We have 4 owner problems that mention the Central Locking Solenoid