Q: I know I should know this already since I know HTML well enough, but what exactly is a markup language?
A: A markup language is simply a set of symbols (in HTML and XML, they are called tags) that convey specific information about the text being marked up. For example, a markup symbol could convey how text is supposed to be displayed or processed, or what the text’s context or purpose is.
In HTML, the markup symbols (or tags) of <strong></strong> tell a browser to display the text inside these opening and closing tags in a bold font face. In XML, the markup symbols (or tags) convey the text’s context or meaning.
By the way, the markup languages of today are based on the concept of marking up a manuscript, derived from the early days of publishing.