Metadata is data about data, data associated with an object, a document, or a dataset for purposes of description, administration, technical functionality and preservation.
There are three important aspects about metadata: framework, encoding and content. Framework is the general structure of the documents, encoding refers to syntactic scheme to write down the content and content is specific information conveyed by the metadata.
There are three main types of metadata: descriptive, structural and administrative metadata.
Descriptive metadata serves the purpose of discovery, identification ad selection of a resource. The descriptive metadata standards includes common ones such as Dublin Core, the most widely used metadata standard for web resources, and TEI, Text Encoding Initiative standard for describing digital humanities and social sciences. It includes many domain specific metadata standards such as AgMES, CSDGM, DwC and EML.
Structural metadata describes the physical structure of resources, and it can be used to describe relationships between components in an object. METS, Metadata Encoding and Transmission standard, is an XML schema for encoding descriptive, administrative, and structural metadata regarding objects within a digital library.
Administrative metadata is related to the use and management of the digital objects, documents, or datasets. It has three types: technical metadata, rights management metadata and preservation metadata. Please refer to Technical, Rights and Preservation Metadata page for more information.