Metadata (or metainformation) is "data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including:
Descriptive metadata – the descriptive information about a resource. It is used for discovery and identification. It includes elements such as title, abstract, author, and keywords.
Structural metadata – metadata about containers of data and indicates how compound objects are put together, for example, how pages are ordered to form chapters. It describes the types, versions, relationships, and other characteristics of digital materials.
Administrative metadata – the information to help manage a resource, like resource type, permissions, and when and how it was created.
Reference metadata – the information about the contents and quality of statistical data.
Statistical metadata – also called process data, may describe processes that collect, process, or produce statistical data.
Legal metadata – provides information about the creator, copyright holder, and public licensing, if provided.
Metadata is not strictly bound to one of these categories, as it can describe a piece of data in many other ways.
Metadata has various purposes. It can help users find relevant information and discover resources. It can also help organize electronic resources, provide digital identification, and archive and preserve resources. Metadata allows users to access resources by "allowing resources to be found by relevant criteria, identifying resources, bringing similar resources together, distinguishing dissimilar resources, and giving location information". Metadata of telecommunication activities including Internet traffic is very widely collected by various national governmental organizations. This data is used for the purposes of traffic analysis and can be used for mass surveillance.
Metadata was traditionally used in the card catalogs of libraries until the 1980s when libraries converted their catalog data to digital databases.