Pin tumbler lock

The pin tumbler lock is the standard, generic key lock. You see pin tumbler locks in action every day; when you unlock your car, or your front door, you're using a variation on the pin tumbler. In fact, the vast majority of key / keyhole locks are of the pin and tumbler variety. (Some may be more advanced, with mechanisms to thwart conventional lockpicks, but the basic workings are the same). They consist of a cylinder (known, in the business, as a plug, and the place where the key goes), a casing for the plug (known as the hull, in which the plug will rotate when the proper key is inserted), and several pins. Essentially, the pins are each broken into two pieces. When the proper key is inserted, the ridges push the pins upward, so that the top half of the pin is in the hull, and only the bottom half remains in the plug. The line between the plug and the hull is known as the sheer line. With only the bottom half of the pin (known as the key pin) still in the plug, it can be freely moved with a twist of the key. The top half of the pin (known as the driver pin) remains in the hull until the plug is back inline, and the key can be withdrawn. They then drop down and prevent the motion of the plug, once more.

    
To Illustrate:
In this figure, we see a pin tumbler lock in the absence of a key.
Note those nasty bold driver pins preventing the movement of our plug!


   
            §   §  §   §   §    <-- springs            
            §   §  §   §   §        
            §   |  §   |   | <-- bold lines represent the driver pins
          __|___|__|___|___|__ <-- This here is the sheer line
            |   |  |   |   |  
            |   |  |   |   |                   
            |   |  |   |   |    <-- Key pins are not in bold
            |   |  |   |   |    
                  
          ____________________

             §   §  §   §   §   <-- springs pushed up by terrific, god-like power of key
             |   |  |   |   |
             |   |  |   |   |    <-- driver pins all above the sheer line
           __|___|__|___|___|__  <-- this is still the sheer line
----------   |   |  |   |   |  
 |--|     :  |   |  |   |   |  <-- key pins all below sheer line 
 |  |     :  ^   |  ^   |   |                   
 |  |     :      ^      ^   ^    <-- the little up-arrows are supposed to represent the ridges of the key
 |--|     :  <-- poorly drawn key, resting in the keyway of the plug         
---------- ____________________

Now, lets picture it looking at one pin, through the pinhole.
As we can see, the plug cannot be turned, as a solid black
driver pin blocks its path.


   §  <-- spring
   §
   | <--driver pin
   |
{-----} <-- sheer line
{  |  }
{  |  }
{  |  } <-- keyway
{  |  }
{     }
{=====}


Now, however, an invisible key has been inserted which pushes our
driver pin up, and out of the way. If I were better with ascii, they keyway
could now be turned. 

   § <-- is still a spring
   |
   | <-- our recently liberated driver pin
   |
{-----} <-- sheer line
{  |  }
{  |<-} - that's our key pin, all on it's lonesome
{  |  }
{     }
{     }
{=====}

For a bit more on pin tumblers, the MIT guide to lockpicking. Please use for good, not evil. If you require any clarification, feel free to /msg me.