XHTML is an application of XML, which is quite a strict angle-bracket language.
HTML is an application of SGML, which is a much less strict angle-bracket language.
(XML is also an application of SGML.)
At one time, people hoped that the solution to the mess of the late 90s web markup was to persuade everyone to write XHTML rather than HTML, perhaps in the hope that the enforced discipline would transform all those polo-necked frameless-spectacle wearing graphic designers into computer programmers. Alas, there was not actually much demonstrable benefit to all this exhausting prostration at the altar of the XHTML validator, so XHTML is now out of fashion and HTML is back in.