Meta data is :

*  Data about Data.
*  Data about Data in software.
*  Data about Programs.
*  Data about Programs.

See dublincore for a set of standard metadata tags appliciable to anything :

The w3c defines metadata here :

""Metadata is machine understandable information for the web. The W3C Metadata Activity addressed the combined needs of several groups for a common framework to express assertions about information on the Web, and was superceded by the W3C Semantic Web Activity."

see "" Missing is a part of the Web which contains information about information - labeling, cataloging and descriptive information structured in such a way that allows Web pages to be properly searched and processed in particular by computer. In other words, what is now very much needed on the Web is metadata. W3C's Metadata Activity is concerned with ways to model and encode metadata. A particular priority of W3C is to use the Web to document the meaning of the metadata. Our strong interest in metadata has prompted development of the Resource Description Framework (RDF ™) and its relative PICS ™ (Platform for Internet Content Selection). PICS is now complete; work on RDF continues.""