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.
HTML stands for Hypertext Markup Language that contain the huge set of markup tags which are very useful for creating a static web pages while XML stands for Extensible Markup Language which is generally used to customized markup languages that defined by users own.