Simply put, metadata is "data about data". This structure has been around for a long, long time, practically from the first libraries. Librarians would catalog the collection to make information more readily available. The card catalog contains elements separate from the record itself., not embedded in the record, but in a separate place. CIP (Cataloging in Publication) is data that is printed on the verso of a book's title page and is embedded in the record itself. This is what occurs in Internet applications of metadata; specifically the metadata tagging structures that we find in the head of our HTML code define the content. The metadata tags are embedded.
So why is this all important? The Internet is a vast, unstructured resource of information. Search engines try to help sort it all out, but without structure, the content is lost. The Internet needs some sort of controlled vocabulary that will define resource descriptions (a schema). Along comes Dublin Core.
Dublin Core is a set (15) elements for describing networked resources. This set of elements attempts to structure data into controlled vocabulary environments. Read the resources below for more detailed information regarding the importance of metadata tagging.