XML is the Extensible Markup Language, a markup language for structured documents. XML is derived from SGML, the Standard Generalized Markup Language; like SGML, XML is a language for defining tags and the relationships among them. It's like a generalization of HTML that lets you define your own tags.
Tags in XML look similar to HTML tags: they consist of a tag name plus optional attributes, surrounded by angle brackets. As with HTML, an opening tag and a closing tag (in which the tag name is preceded by a slash) are used to mark the content between the tags. One syntactical difference is that in XML, an empty tag (one that contains no content, such as <img>) must end with a slash just before the closing angle bracket. For example, an <img> tag in XML might look like this:
<img src="test.png" />
For more information about XML, see http://www.xml.com.