What is Metadata?
Metadata is "data about data". For a phone call, that may include the identity and location of the two ends, the time of day and duration - but not the contents of the call itself. On the Internet, the line between metadata and data is blurry.
In public policy, the concept of metadata is most relevant when it is distinguished from the data which it describes, and different laws, rules or standards are applied to the two. However, for Internet communications, the distinction is often not a clear one, because the metadata is part of the data.
If we take an email message as an example, then the date it was sent, and the people who sent and received it may be classified as metadata. But these data fields are not separated from the email by anything more than a blank line. It is all, data and metadata, sent and received as one data stream. They are not recorded and processed by a separate system, as is the case for the metadata about a telephone call.
Bulk metadata analysis is an extremely powerful way of determining people's social networks, and inferring facts about them. For example, the details of who is friends with whom on a social network may be considered metadata - and yet it is possible to make a very accurate guess about a person's sexuality from that information alone.