This is known in SGML as a Processing Instruction, or PI. HTML is an application of SGML, while XML is a subset of SGML. XHTML is an application of XML.
A PI doesn't mark up content like a normal tag. It is used for specifying instructions to the user agent's parser.
You also have directives, which in SGML are surrounded by
<!...>. The doctype declaration is an example. Directives can also contain comments, surrounded by pairs of double hyphens. So if you want a comment in HTML or XML you can write
<!--comment-->. Another example is CDATA sections, marked up with