XML (Extensible Markup Language) is a standard defined by the World Wide Web Consortium (W3C). XML uses tags (identifiers enclosed in brackets, for example, <para>) to label content in text documents. These tags, collectively called the "markup," can be read by humans and computers. XML tags, like SGML tags, encode not only the meaning of the content but also its structure.

XML looks like HTML but XML has nothing to do with the display of the content, which is the central focus of HTML. The following XML tags label the content they enclose as a paragraph.

<para>This is a paragraph.</para>

The <para> tag has nothing to do with the display of the sentence. The tag, for example, does not take attributes such as font, size, or style.

Unlike HTML, there is not a single set of tags used in all XML documents. XML enables designers to create their own set of tags that are appropriate to their business. For example, Product Advertising API encodes its APIs using an XML document called a WSDL. Other companies create their own XML tags to define their APIs. The tags in these two WSDLs most likely would be completely different. The syntax and rules by which the XML elements are defined in the WSDL, however, are the same. Because a web server handles multiple requests that implement multiple WSDLs, each request must specify the WSDL it is using..