Metadata

Metadata (or "data about data") is everything about a piece of information, apart from the information itself. So the content of a message is not metadata, but who sent it, when, where from, and to whom, are all examples of metadata. Legal systems often protect content more than metadata: for instance, in the United States, law enforcement needs a warrant to listen to a person's telephone calls, but claims the right to obtain the list of who you have called far more easily. However, metadata can often reveal a great deal, and will often need to be protected as carefully as the data it describes.