Metadata is defined as data about data.  For data (or any information) to be understood and trusted we must understand the vocabulary, structure and source of that information - this is the informations metadata.  Metadata comes in two promary forms; "Meta models" which define the structure and meaning of the information and "instance metadata" which defines information about a particular piece of information - such as the autor or creation date.  Both forms of metadata are sored in multiple formats and standards, including RDF (from the W3C) and the MOF From the OMG.