A markup language is one that is designed for defining and presenting text. HTML (HyperText Markup Language), is an example of a markup language.
Within a text file such as an HTML file, elements are marked up using tags which explain the purpose of that part of the content.
Used by traditional word processing system with WYSIWYG (what you see it is what you get); this is hidden from human authors, users and editors.
Combined with text to provide instructions on text processing to programs. This text is visibly manipulated by the author.
Labels sections of documents as to how the program should handle them. For example, the HTML <td> defines a cell in an HTML table.