Jon Udell at InfoWorld - Managing metadata:

"Everyone knows the common definition: Metadata is data about data, a secondary thing that's separate in some way from the primary thing to which it refers. But that definition begs a series of questions. Is metadata something we derive from data, or assign to it? Does it classify things, or enable us to search for things, or govern the behavior of things? If data that is described by metadata also, in turn, refers to other data, does it then qualify as both data and metadata?

These questions can verge on the philosophical, but by working through some examples, we can define various types of metadata, list the benefits that we expect from using it, and identify the challenges associated with maintaining it. Programs, documents, messages, files, Web resources, and Web services are some of the IT constructs often described by metadata. Let's review the roles that metadata can play in these different scenarios."