Too long; didn't read summary: Skeleton keys do exist, they are keys that have been filed down to bypass the wards in warded locks, thereby opening all locks of that type.
When I was 8 years old in a Meijer's Thrifty Acres store I wandered into an independent key shop they had inside the store. Among the racks of shining keys was a single peg of old-fashioned looking keys (bit keys actually) that were labelled "skeleton keys". I don't know exactly where I had heard the word before, but I had a firm idea of what a skeleton key was - a key that would open any lock!
Gasp! Could they really sell something like this to just anyone!? If I bought it could I open any lock? Even at 8 I was pretty dubious. First off, the thing they had hanging up didn't look like it would fit into any lock that I had ever seen and second, geez, why would anyone want to buy a lock that any schmuck could open with a skeleton key from Meijer's?
I was right to be skeptical. In actuality, there's not much a skeleton key can open - only the simplest and least secure locks of yesteryear - warded locks.
Warded locks have been around forever, and they work on a very simple premise: there is a mechanical device (usually a latch or bolt) that must thrown (moved) in order to actuate the lock (a sliding bolt on a gate is a very simple example of this). The mechanical device is hidden inside the lock body and requires a tool, ie the key, to be inserted through a hole in the lock body then turned to throw the lever or bolt directly (or a latch attached to the lever or bolt), thus releasing the lock.
What makes these locks "warded" is that barriers (wards) have been put into place inside the lock to prevent just any old key from being able to turn inside the lock. Each lock has wards located in different points along the turning radius so one key can't open a different lock of the same type.
A Modern Example
These types of locks are still in use today and are as close as your local hardware store. For a demonstration I've pulled apart a warded Master lock. Slideshow below:
So what is a skeleton key? A key that has been filed down to just its bare bones. Get it? It's the skeleton of a regular key. In the above example, the only part of the key that needs to be "wide", that is, uncut, is the tip. By taking a key packaged with one of these locks and filing it down along the entire length, minus the tip, the key will turn in ALL warded master locks. Thus the warding plates inside the lock, no matter where they are placed, are avoided altogether.
Sandy Eisele is owner of Peninsula Locksmiths and loves to talk and write about all things lock related.