The Extensible Markup Language (XML) is a set of rules for encoding documents in machine-readable form. XML was designed to transport and store data, whereas HTML was designed to display data. XML documents consist of information wrapped in user-defined tags; this is one of the main differences between HTML and XML; in HTML, tags are pre-defined, whereas XML allows the users to define her own tags and document structure.
An XML schema describes the structure of an XML document. According to Wikipedia, there are languages developed specifically to express XML schemas; one of the more widely used languages used to express XML schemas is XML Schema (with a capital “S”). In other words, this can get pretty confusing pretty quickly.
XML standards include specifications that aim to ensure consistency across XML documents. Examples include Namespaces in XML, which provides a mechanism for the names of elements and attributes in XML documents; XML base, which provides a way of associating XML elements with uniform resource identifiers, and XML Inclusions, which provide a system for merging XML documents.
These technologies have to do with the carrying of data across the web; essentially, XML is about communicating the contents of data, whereas HTML is about the layout of the content. I think, anyway. These technologies quite literally require a language of their own.
