XML (Extensible Markup Language) is a flexible way to create 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 is currently a formal recommendation from the World Wide Web Consortium as a way to make the Web a more versatile tool. XML is similar to the language of today's Web pages, HTML, in that both contain markup symbols to describe the contents of a page or file. HTML, however, describes the content of a Web page (mainly text and graphic images) only in terms of how it is to be displayed and interacted with. For example, a <P> starts a new paragraph. XML describes the content in terms of what data is being described. For example, a <PHONENUM> could indicate that the data that followed it was a phone number. This means that an XML file can be processed purely as data by a program or it can be stored with similar data on another computer or, like an HTML file, that it can be displayed. For example, depending on how the application in the receiving computer wanted to handle the phone number, it could be stored, displayed, or dialed.
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.
Early applications of XML include Microsoft's Channel Definition Format (CDF), which describes a channel, a portion of a Web site that has been downloaded to your hard disk and is then is updated periodically as information changes. A specific CDF file contains data that specifies an initial Web page and how frequently it is updated. Another early application is ChartWare, which uses XML as a way to describe medical charts so that they can be shared by doctors. Applications related to banking, e-commerce ordering, personal preference profiles, purchase orders, litigation documents, part lists, and many others are anticipated.
X.400 is the messaging (notably e-mail) standard specified by the ITU-TS (International Telecommunications Union - Telecommunication Standard Sector). It's an alternative to the more prevalent e-mail protocol, SMTP. X.400 is common in Europe and Canada. It's actually a set of standards, each in the 400-number range.
Because X.400 stipulates a number of possible address characteristics that SMTP does not, an X.400 address can be long and cumbersome. On the other hand, X.400 adherents note that it is an official standard whereas SMTP is a "de facto" standard. Thus, products with X.400 implementations can be tested more rigorously than products with SMTP implementations can. X.400 offers more capabilities than SMTP does. However, many of these capabilities are seldom used.
An SMTP e-mail address that looks like this hypothetical address:
might look like this in an X.400 e-mail message:
G=Georg; S=Hansen; O=sintef; OU=delab; PRMD=uninett; ADMD=uninett; C=no
X.400 is a complex standard that is difficult to summarize here. The arguments in favor of and against each of the opposing e-mail standards are also complicated.