(EXtensible Markup Language) An open standard for describing data from the W3C. It is used for defining data elements on a Web page and business-to-business documents. XML uses a similar tag structure as HTML; however, whereas HTML defines how elements are displayed, XML defines what those elements contain. While HTML uses predefined tags, XML allows tags to be defined by the developer of the page. Thus, virtually any data items, such as "product," "sales rep" and "amount due," can be identified, allowing Web pages to function like database records. By providing a common method for identifying data, XML supports business-to-business transactions and has become "the" format for electronic data interchange and Web services (see XML vocabulary, Web services, SOA and EDI).
XML Is Only a Format
Since its introduction in 1998, XML has been hyped as the panacea to e-commerce, but it was only the first step. The human-readable XML tags provide a simple data format, but the intelligent defining of these tags to serve business needs properly and everyone's adherence to using the same tags determines the real value of XML. Countless vocabularies have been developed for vertical applications; so many in fact, that a universal language was developed to provide a standard for interoperability between them (see UBL).
XML Documents Can Define Themselves
An XML document can include a self-describing set of rules that identify the tags and their relationships; for example, only one XYZ tag is allowed within an ABC tag, or there must be one XYZ tag within every ABC tag and so forth. The rules are called a "schema" (see XML schema).
More Rigid than HTML
Unlike HTML, which uses a rather loose coding style and which is tolerant of coding errors, XML pages have to be "well formed," which means they must comply with the rules. See XSLT, DTD, DOM, XHTML, HTML, SGML, SMIL and XML-RPC.
XML tags define the data, whereas HTML tags define the page layout, but the tag structure is the same. Text elements are wrapped in "open" and "close" tags, also called "start" and "end" tags, which begin with the less-than (<) character and end with greater-than (>). The HTML in this example defines font size and makes the date boldface. Note that all close (end) tags contain a slash (/) after the beginning less-than (<) character.
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