What Is A Wafer-Tumbler Lock?
These work the same basic way as pin-and-tumblers, but they have thin wafer-shaped tumblers rather than pins. You pick the wafers exactly the same way you pick pins -- in fact, it is a little bit easier to pick wafer-tumbler locks because the keyhole is wider.
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. To pick these locks, you work the wafers on both sides as you apply pressure with the tension wrench. Wafer locks are found in most filing cabinets, lockers and cars, as well as in many padlock designs.
Credit to Howstuffworks.com