In "Make Friends with Metadata," published this week on Law.com, Craig Ball describes two types of metadata: application metadata and system metadata.
Application metadata is information typically absent from the printed page and embedded in the file it describes, moving with the file when you copy it. It has a fearsome reputation among lawyers because of its nasty habit of carrying sensitive information, such as deleted text, and who else has seen the document -- but it's that very capacity for holding more than meets the eye that enhances its evidentiary value.
By contrast, system metadata isn't embedded in the file it describes, but stored externally and used by the computer's file system to track file locations and store demographics. A file's name, size, location, path and dates of creation, modification and access are common system metadata fields. Not all metadata is embedded for the same reason that cards in a library card catalog aren't stored between the pages of the books. Having both application and system metadata is advantageous because, when metadata is stored both within and outside a file, discrepancies can expose data tampering.
Why is metadata important at all? Ball reminds readers, "The lessons of paper discovery still guide us in the digital labyrinth. Because application metadata is part of the file, there's an obligation to produce it commensurate with the obligation to produce, e.g., marginalia on paper documents."
Ball's article is another good source for those learning about metadata.