Mark-Up Languages use plain text, adding commands in their own syntax to describe it further. These commands are called "tags". While the part in plain text part is the content, the tags are responsible for the formatting. 

Word Processor Programs are based on such a Mark-Up-Language (XML). Users of these programs do not necessarily have to think about XML.

Another common Mark-Up-Language is HTML (Hypertext Markup Language), which is used for Websites. A .html-file is usually opened with the preferred web browser. The browser is interpreting the tags and just showing the content. To see both - content and tags - the file has to be opened in a text editor.

The typesetting system TeX is also similar to a Mark-Up Language. It is often used in fields of natural science, because it was primarily developed to depict mathematical formulas accordingly.