XML stands for eXtensible Markup Language.
It is a way of organizing text and/or contents with tags. Tags look like this:
<sometag>some content</sometag>
Of course, although possible, this is not designed to be read by humans. It is designed to be processed by machines. Typically one program generates an XML document and another one computes it to some further extent.
And XML document very much looks like an HTML document, except that the tags are different.
There are actually many different dialects of XML, each for a different purpose. Each of these dialects has its own tags. And even more tags can be added. Hence the “eXtensible” in the name.
There is an XML dialect for writing web pages. It is called XHTML. It is basically the same thing as HTML except for minor syntaxic differences. For example in HYML you would write <BR> whereas in XML you would write <br />.