Essentially, metadata is data about a digital object. The metadata is usually provided by the creator or distributor of the object, and often either accompanies the object or is embedded in the file header. As such, metadata can be very useful as the basis for information storage and retrieval systems. 

The term metadata has been used only in the past 15 years, and has become particularly common with the popularity of the World Wide Web. But the underlying concepts have been in use for as long as collections of information have been organized. Library catalogs represent a well established variety of metadata that has served for decades as collection management and resource discovery tools.

Data which provides information about a resource.

Metadata is data about data. The term refers to any data used to aid the identification, description and location of networked electronic resources. Many different metadata formats exist, some quite simple in their description, others quite complex and rich. 

It is inherent in the concept of metadata that there is an association of some kind between the metadata and the information resource which it describes. For example, a library catalogue record is a collection of metadata elements, linked to the book or other item in the library collection through the call number. Information stored in the "META" field of an HTML Web page is metadata, associated with the information resource by being embedded within it. The indexing data held by Web crawlers is also metadata (though not very good metadata) - linked to the information resource through the URL.

Metadata can be an information resource in its own right. For example, a review of a film - which on one level is a piece of metadata related to the film - is, on another level, a literary work with its own author and perhaps its own intellectual property constraints.

In recent years there has been a focus on metadata in relation to those information resources which can be accessed through the World Wide Web. I propose to concentrate in this paper on that aspect of metadata, and to discuss the Dublin Core metadata standard in particular. However, we should remember that there are other metadata schemes which are in use in relation to the Internet, and that metadata has a flourishing existence outside the Internet context. The huge amounts of cataloguing and indexing data created over many decades by the library community, and the Abstracting and Indexing community, are equally entitled to be described as metadata.

Metadata is data about data. It describes the attributes and contents of an original document or work. 

Metadata is structured data about data. Increasingly the term refers to any data used to aid the identification, description and location of networked electronic resources. In this context there now exists a variety of metadata formats from the basic proprietary records used in global internet search services, through a continuum encompassing simple attribute/value records such as the ROADS templates used in eLib subject services, the more structured TEI and MARC formats, and at the richest level detailed formats such as CIMI and EAD, typically applied to archival material.

Metadata is data which describes attributes of a resource. Typically, it supports a number of functions: location, discovery, documentation, evaluation, selection and others. These activities may be carried out by human end-users or their (human or automated) agents.

• intellectual property rights
• including the contractual terms related to the document's use and distribution
• electronic commerce
  to encode prices, terms of payment, etc.
• content rating
  
to disclose the nature of a particular page's contents which can be used in filtering content so that, for example, parents can block inappropriate material from children
• digital signatures
  
can you trust this document? 
• privacy issues
  what information do browsers collect when you visit? what information are users willing to disclose about themselves when visiting a Web site?

Metadata is machine understandable information for the web

There is now a wealth of information on every subject available on the Net. For many, however, the true excitement of the Web is in the services that you can access from your home or office. Today's Web gives people access to news, to the weather and to financial services. Via the Web, users can purchase books, computers, clothes, and any number of other items; you can book seats on planes and rooms in hotels. The possible uses of the Web seem endless, but there the technology is missing a crucial piece. Missing is a part of the Web which contains information about information - labeling, cataloging and descriptive information structured in such a way that allows Web pages to be properly searched and processed in particular by computer. In other words, what is now very much needed on the Web is metadata. W3C's Metadata Activity is concerned with ways to model and encode metadata. A particular priority of W3C is to use the Web to document the meaning of the metadata.