The term markup language usually refers to a set of characters that a writer inserts into a file of text to indicate some sort of special meaning or action to be taken at a point in a documentation.
For example, one type of markup might denote semantic meaning whereby a text has notations indicating here is the title of the document, here is begins the next chapter, here is the beginning of a new paragraph, etc.
A very common type of markup is presentation markup. This is the type of markup we use on this wiki. With presentation markup, you indicate what special actions should be taken during transformation of the text from raw text to the form presented to the user. Here on the wiki, we can
and more.
Typically, if one has a semantic markup, one also has some sort of means to also transform the semantical parsed text into a preferred presentation format. However, the opposite is not often true - if one only has presentation markup, it is quite hard to generate an accurate semantical tree of the document, unless the user takes great care to use presentation and content in such a way that one can somehow derive semantical meaning from the text.
These distinctions are sometimes summarized as being between procedural markup and declarative markup.
See Simple Markup for a simple presentation markup language.