Markup languages are designed for the processing, definition, and presentation of text. The language specifies code for formatting, both the layout and style, within a text file. The code used to specify the formatting are called tags. HTML is an example of a widely known and used a markup language.
A markup language is a language that annotates text so that the computer can manipulate the text.
Most markup languages are human readable because the annotations are written in a way to distinguish them from the text. For example, with HTML, XML, and XHTML, the markup tags are < and >. Any text that appears within one of those characters is considered part of the markup language and not part of the annotated text.
When you format text to be printed (or displayed on a computer or other device screen), you need to distinguish between the text itself and the instructions for printing the text. The markup is the instructions for displaying or printing the text.