Back in the days of warded locks (where the wrong key wouldn't turn because it would run into obstructions, and the right key had cut-outs to slide past those), a "skeleton" key was a kind of lockpick, where as much as possible had been cut away in the hope that what was left would be small enough to slide past the ward yet large and strong enough to operate the mechanism. Even for common locks, a set of these was often required in order to handle all the possibilities. These days, there mostly isn't such a thing. The only time you'll be likely to see a skeleton key is if your house still has old warded locks on a few of its doors (typically interior doors such as bedrooms and closets), the original keys have been lost, and someone bought a skeleton key as a sloppy alternative to having someone figure out what the proper key(s) should be. As this implies, warded locks aren't very secure, by today's standards -- which is why they've almost completely been replaced by lever-tumbler or pin-tumbler locks.