The Windows Registry
The windows registry is a hierarchical tree database of data that every windows machine has.
It’s a nightmare and as a responsible windows programmer you should never use it.
The problem is applications can fill the registry with garbage which:
Consumes disc space and resources
Makes it possible for new applications to be broken by garbage left by old applications.
Don’t use it!
See local storage can be dangerous for an analogous problem in the web world although it’s worse in the windows registry since the customer has no way of really know what application uses what information in the registry.