Where does XML come from?
In a nutshell, XML was first developed to make SGML usable for Web purposes. SGML is a very large and complicated norm (thousands of pages). XML is kind of a subset of SGML, but at the same time has more restrictive rules, so that parsers and transformers working on XML are faster and more easy to implement.
XML is a W3C recommendation to which are attached other recommendations as XPath, XPointer, XLink, DOM, XSL, SVG, etc.