##metadata##
[Wikipedia 2014-10-05] "data about data". The term is ambiguous, as it is used for two fundamentally different concepts (types). Structural metadata is about the design and specification of data structures and is more properly called "data about the containers of data"; descriptive metadata, on the other hand, is about individual instances of application data, the data content. The main purpose of metadata is to facilitate in the discovery of relevant information, more often classified as resource discovery. Metadata also helps organize electronic resources, provide digital identification, and helps support archiving and preservation of the resource. Metadata assists in resource discovery by "allowing resources to be found by relevant criteria, identifying resources, bringing similar resources together, distinguishing dissimilar resources, and giving location information".
[BG] Usually summarised as "data about data". For instance, (to take a non-genealogical example that I hope still applies), the metadata about a railway wagon number on American railroads tells us that it is up to ten characters long, consisting of a group of up to four letters followed by a group of up to six numbers. It also tells us something about the meaning of those letters (usually ownership). Knowing the metadata helps a programmer write basic validation on input and understand at least some of what the data means. Much of the GEDCOM standard consists of metadata describing how big data items can be, what they mean, what they relate to, etc.