Aren't XML, SGML, and HTML all the same thing

Aren't XML, SGML, and HTML all the same thing?

Answer:

Not quite; SGML is the mother tongue, and has been used for describing thousands of

different document types in many fields of human activity, from transcriptions of ancient
Irish manuscripts to the technical documentation for stealth bombers, and from patients'
clinical records to musical notation. SGML is very large and complex, however, and
probably overkill for most common office desktop applications.
XML is an abbreviated version of SGML, to make it easier to use over the Web, easier
for you to define your own document types, and easier for programmers to write
programs to handle them. It omits all the complex and less-used options of SGML in
return for the benefits of being easier to write applications for, easier to understand, and
more suited to delivery and interoperability over the Web. But it is still SGML, and XML
files may still be processed in the same way as any other SGML file (see the question on
XML software).
HTML is just one of many SGML or XML applications—the one most frequently used
on the Web.
Technical readers may find it more useful to think of XML as being SGML-- rather than
HTML++.