1. What are padlock shims?
A= Padlock shims are formed pieces of thin yet stiff metal designed to take advantage of mechanical imperfections and tolerances in the latching design of some popular brands/models of locks. In other words, they are a bypass tool, not a pick.
You can see what they look like here: http://www.lockpickshop.com/page/LPS/PROD/SPT/SPS-20
2. What do they do?
A=By slipping a shim between the (toe, heel, or both as required) and the actual latching bar of the padlock, it will allow the shackle to be pulled out of the lock without the use of a key or pick.
3. Do they work on all padlocks?
A=No. Many of the popular brand of quality padlocks now use a double-ball locking system that will actual wreck the shim if used. On old Master padlocks, and on some newer no-name or hardware brands this bypass technique may still work if the latching bar system is used.
4. Why do most places sell them in large packs, aren't they resuable?
A= Yes, they are reusuable, but since they are made of thin metal and may get damaged during normal use, they are generally considered a throw-away item. That is, if you get more than 5 uses out of a single shim you are probably fortunate.
5. How do I tell if they have a locking ball system?
A= Open the lock and look into the hole the shackle sits in. You'll be able to tell immediately just by looking.
Hope this helps those of you considering purchasing these.
Enjoy.