Extensible Markup Language File (XML)

XML (Extensible Markup Language) is a flexible way to create common information formats and share both the format and the data on the World Wide Web, intranets, and elsewhere. For example, computer makers might agree on a standard or common way to describe the information about a computer product (processor speed, memory size, and so forth) and then describe the product information format with XML. Such a standard way of describing data would enable a user to send an intelligent agent (a program) to each computer maker's Web site, gather data, and then make a valid comparison. XML can be used by any individual or group of individuals or companies that wants to share information in a consistent way.

XML has been designed to be usable on browsing platforms, while full-fledged SGML is usually more suitable for authoring platforms. In fact, XML has for most purposes become the only form of SGML that is suitable for public sharing. Many SGML languages1 that are realistically suitable for authors admit rapid automatic translation to nearly equivalent XML languages.

Difference Between file XML and HTML

XML was designed to transport and store data, with focus on what data is.

HTML was designed to display data, with focus on how data looks.

In HTML most paragraphs are marked up using an opentag "<P>" at the beginning of the paragraph without needing a closetag "</P>" at the end, while there must be a closetag for every opentag in XML.

In HTML tag names are not case-sensitive, while in XML tag names are case-sensitive.

In HTML some attribute values need not be placed inside quotation marks, while in XML all attribute values must be quoted.

Building blocks of XML documents are nested, tagged elements. Each tagged element has zero or more subelements, zero or more attributes, and may contain textual information (data content). Elements can be nested at any depth in the document structure. Attributes can be of different types, allowing one to specify element identifiers (attributes of type ID), additional information about the element (e.g., attributes of type CDATA containing textual information), or links to other elements of the document (attributes of type IDREF(s)/URI(s)).

The Owner specifies access control policies and sends the Publisher the documents it is entitled to manage. For each document, the Owner sends the Publisher also information on which subjects can access which portions of the document, according to the access control policies it has specified. All this additional information is encoded in XML and attached to the original document, forming the so-called security enhanced XML document. The information contained into the security enhanced XML document allows a subject to verify the authenticity of the information returned by the Publisher, but it is not sufficient to make a subject able to verify the completeness of a query result. For this reason, the Owner supplies the Publisher with some additional information about the structure of the original XML document. This information is encoded into an XML document, called secure structure, which contains the structure of the XML document, transformed through the use of an hash function.

XML is now as important for the Web as HTML was to the foundation of the Web. XML is everywhere. It is the most common tool for data transmissions between all sorts of applications, and is becoming more and more popular in the area of storing and describing information.

XML is "extensible" because, unlike HTML, the markup symbols are unlimited and self-defining. XML is actually a simpler and easier-to-use subset of the Standard Generalized Markup Language (SGML), the standard for how to create a document structure. It is expected that HTML and XML will be used together in many Web applications. XML markup, for example, may appear within an HTML page.