Metadata is structured data that may be used to describe characteristics pertaining to a resource. The metadata of a resource will consist of a number of elements that represent attributes of that resource. These elements may have one or more values.
Metadata was used extensively in the HTML of early web pages and search engines would rely on terms included in the metadata of pages. People abused the feature by including terms that were unrelated to the content of the page in order to draw more traffic and search engine developers subsequently lost faith.
An example of a metadata schema is DublinCore.