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HTML is a Markup Language, which uses a fixed set of tags (elements).  This means that if you don't find the tag you need, you're out of luck, because there is no mechanism to add tags to the language (short of a very lengthy and complex standards process).

HTML also mixes the content with the layout.  While the use of CSS goes some way towards separating the two, it isn't easy to completely separate them because of browser CSS support deficiencies, and because it's not easy to learn how to use such technologies to full advantage.

Another limitation of HTML is that there is no semantic content provided by the available tags.  For example:

<h1>Sir Arthur Conan Doyle</h1>
<h2>The Hound of the Baskervilles</h2>
<h2>Sherlock Holmes</h2>

does not convey the fact that we're talking about a book, its author, and its hero.  The <hn> tags relate only to how the text is displayed in a browser.

In contrast to HTML, XML (eXtensible Markup Language), is a Meta-Markup Language.  That means that the tags are not predefined;  you can invent your own tags to solve your particular problem, without being restricted by arbitrary standards processes, etc.

XML is purely about content, and adding semantics (meaning) to that content. So we can restate the above book-related HTML in XML:

<book>
  <title>The Hound of the Baskervilles</title>
  <author>Sir Arthur Conan Doyle</author>
  <hero>Sherlock Holmes</hero>
</book>

and in the process add the necessary semantic information.

So long as we construct our XML following a well-defined and mutually understood set of rules, we can convey this semantic information to others (people, or computer applications of all sorts).

XML has become amazingly popular, and is used in many, many different applications, for many different purposes.