Metadata is data about data. An example is a library catalog card, which contains data about the nature and location of a book: It is data about the data in the book referred to by the card.

The content combined with its metadata is often called a content package.

Uses

Metadata has become important on the World Wide Web because of the need to find useful information from the mass of information available. Manually-created metadata adds value because it ensures consistency. If one webpage about a topic contains a word or phrase, then all webpages about that topic should contain that same word. It also ensures variety, so that if one topic has two names, each of these names will be used. For example, an article about sports utility vehicles would also be given the metadata keywords ‘4 wheel drives’, ‘4WDs’ and ‘four wheel drives’, as this is what they are known as in Australia, South Africa, and Namibia.

For examples of metadata for an audio CD, look at the MusicBrainz project, or AMG's All Music Guide. Similarly, MP3 files have metadata tags in a format called ID3.

Metadata is more properly called ontology or schema when it is of broad or narrow utility. Both terms describe “what exists” for some purpose or to enable some action. For instance, the library card’s minimal schema enables a user to determine quickly whether they wish to look at the book, and if so, how to find it quickly.

Filesystem metadata

Some file systems keep metadata about files out-of-band, while others keep it in the filename or inside the file. Examples of metadata are the filetype, abstracts, and icons. Examples of filesystems that keep metadata separate but related to the actual data are the BeFS of BeOS, HPFS of OS/2, HFS/HFS Plus of Mac OS, ReiserFS of Linux, NTFS of Windows, and ODS-5 of OpenVMS.

The next major releases of Mac OS X (Tiger) and Microsoft Windows (Longhorn) were expected to include new features that would allow greater flexibility in the maintenance and searching of metadata. Apple has announced that it plans to release Tiger with its Spotlight technology on the April 29 2005, however on August 27 2004 Microsoft announced that when the client and server versions of Longhorn ship in 2006 and 2007, its new filesystem WinFS will only be available as a beta product.

Metadata continues to expand beyond database technologies in the corporation. Today, Metadata is about describing the technical assets of the corporation which can include web services, patterns, frameworks, schemas, components, and any other technology within the organization.

Program metadata

Most executable file formats include metadata describing issues that need to be considered by the runtime or operating system when executing the program.

In DOS, the COM file format does not, but the EXE file format does, and the latter is expanded for Windows to the PE format.

In the Microsoft .NET executable format, extra metadata is included to allow reflection at runtime.

For a list of executable formats, see object file.

See also

• Dublin Core
• ID3
• APEv2 tag
• Exif
• IPTC (image meta-data)
• Kendra initiative
• meta tags
• MOF
• RDF
• The Semantic Web
• Topic maps
• XML
• Magic number (programming)
• metatable
• GNUBrain

External links

• "Metacrap" - An opinion on the limitations of metadata
• Research Paper on Metadata Communications

Blog link

• Enterprise Metadata