Quick Definition
Personally Identifiable Information is personal or biographical data detailed enough to enable a stranger to identify or locate a particular individual and to learn otherwise private things about them.
The public release of personally identifiable information is risky to users because this information can often be used as proof of identity, both online and offline, as well as to obtain government services, engage in financial activity, participate in various forms of "identity theft" targeting the user, or other illicit and possibly dangerous activty.
The safeguarding of personally identiable information is a key object of many privacy and security regulations, and the most damaging consequences of data breaches, both organizationally and individually, often result from the release of personally identifiable information into the wild.