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 more easy than SGML

XML was aimed to Web applications but proved useful for plenty of other applications

XML is a W3C recommendation to which are attached other recommendations as XPath, XPointer, XLink, DOM, XSL, SVG, etc.