You'll find this sort of lock everywhere, from houses to padlocks. They are so popular because they are relatively inexpensive but offer fair security. When you consider the number of lock companies and lock designs, the chances of a criminal having the same key as you is fairly remote.
Some designs use single wafers rather than wafer pairs.
These wafers are spring loaded so that they extend out of the cylinder, binding with the lock housing. The wafers have a hole in the center that the key will fit through. The correct key pulls the wafers down just enough so that they are all retracted into the plug. The incorrect key will either pull the wafers down only part of the way or will pull them down too far, causing them to extend out the other side of the plug. Double-wafer locks have wafer tumblers on both ends of the plug.