All too often, people refer to metadata as "data about data". But while this literal definition is occasionally useful, the line between data and metadata gets blurred too often.

For example, take an object catalogued by a museum.

• Is its height and width... data or metadata?
• Is its barcode... data or metadata?
• Is the donor of the object... data or metadata?

Karen Coyle has a beautiful definition of metadata (you should read the whole article, it's really good):

What is metadata?
 

Metadata is ...
constructed... (Metadata is wholly artificial, created by human beings.)
for a purpose... (There is no universal metadata. For metadata to be useful it has to serve a purpose.)
to facilitate an activity. (There's something that you do with metadata)

Using this rule, we can now clearly distinguish answer the questions posed above.

Height and width are data ... they are an inherent property of the object.
The barcode is metadata ... a construct designed to aid storage and retrieval.
The donor is also data ... since the donor is a fact rather than an artificial construction.

Thanks Karen! You've resolved one of my niggling questions about where the distinction lies.