A skeleton key is a type of key that opens many different locks. The shape of the keys themselves gave rise to their name. Skeleton keys typically have a large open circle at the top, like a head, and a long thin "body" leading down to the key bits at the bottom. Skeleton keys can only open lever locks, or certain types of warded locks.
Skeleton keys open warded locks by having the majority of the key removed, so that it can't interfere with any wards that are in place. In order to prevent skeleton keys from allowing illegal entry, lock manufactures began to add wards around the outside of the lock as well as in the center. This prevented the successful use of skeleton keys. Skeleton keys of this sort are no longer in common usage, other than in furniture such as china cabinets.
Skeleton keys, when used to open lever-type locks, are often known as "master keys." In lever-type locks, there is typically a combination of several levers, and wards. The levers and wards only meet with the key on the sides. The top of the key pushes the levers to the right height, which allows the key to pass through the lock and rotate to open it. A locking system that allows master keys has equal heights on all levers in each lock, but a different set of wards for each door. This allows each door to have unique keys, and remain secure, while a master key can still open each lock. The master key, in this system, has no warded section, and simply opens the door by means of the levers. Master keys are still in common usage, particularly by locksmiths and any business with a large number of locking doors, such as a hotel or resort.