Metadata is simply data that provides information about other data. It is generally used to provide additional information about a piece of data's content. Web pages include metadata about their content in the form of <meta> tags that are used to convey information about the pages description, author, keywords, and more. Metadata contained in HTML is not visually shown on a page but is machine-parsable to provide a web browser with information related to the site's content.
The main types of metadata are descriptive, structural, administrative, reference, and statistical.