HTML is a set of tags to describe a web page.

XML is a set of tags to describe any given type of hiercharchal data, depending on the schema for that data.

XHTML is an XML-compliant implementation of HTML.

So with XML, you could invent your own tags and properties to describe any package of data. You would create a schema that described your legal tags and properties. Then if enough people liked it, you could start exchanging XML data in your format, and possibly make it a standard.

That's what XHTML is - a standardized XML format/schema created for web pages.

All "ML" formats have SGML as the predecessor.