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.

Types of markup language

Presentational Markup:
Used by traditional word processing sytem with WYSIWYG (what you see it is what you get); this is hidden from human authors, users and editors.
Procedural Markup:
Combined with text to provide instructions on text processing to programs. This text is visibly manipulated by the author.
Descriptive Markup:
Labels sections of documents as to how the program should handle them. For example, the HTML <td> defines a cell in a HTML table.