Metadata is, in some sense, data about data, which gives context to the content of your personal information.
For example, metadata is not the call that you made to your friend, but bits of information that are recorded about your call to your friend such as time of the call, length of the call, source number, destination number, approximate location of the caller or receiver and so on.
Metadata is very powerful, often even more powerful than the personal information it describes. Metadata, when aggregated and analysed, can offer insight into behaviour and patterns of an individual, often to a degree which even the target individual is no aware of. Metadata can often describe you in a very complete way and offer information about you which you were not even aware you were revealing.
Demonstrating the power of metadata[edit | edit source]
Imagine the following situation.
You know that 'Person A' is 17 year old female, who's phone spends 1 hour in a church every Sunday, and who has been exchanging messages regularly with 'Person B', 17 year old male. One day, the number of messages has increased, with messages becoming shorter and being sent more often. The there was a long call between Persons A and B, following by a call to Planned Parenthood. This is the sort of information that your phone provider has on you, and can trivially access.
Knowing nothing about the people involved, and having no access to the contents of their messages and calls, it is possible to reasonably infer that the individuals are in a relationship (especially if you can check this data against, let's say, Facebook), the and that the Person A lives in a religious household or is religious herself. The couple has experienced an emotionally charged event, likely a pregnancy scare, and is considering their options.
Being able to make such inferences gives you an insight into a person's life. Almost all service providers retain metadata by default, and the provider may simply sell this data to an advertiser (such as an anti-abortion group) who may target the Person A with their message.
In the example above in strictly conservative countries such as Saudi Arabia, detection of such pattern of metadata may trigger an alert to the woman's parents, which can result in a slew of outcomes detrimental to the Person A's well-being.
All this without any insight into contents of the messages and calls.
See also: w:metadata