These are formats for text exchange. The most well known is HTML, Hypertext Markup Language, used for distributing texts via the Internet. SGML is Standard Generalized Markup Language, used by publishers and the BNC; XML is Extensible Markup Language, intermediate between the other two.
All these standards use plain text with additional extra tags, mostly angle-bracketed, such as <h1> and </h1>. The point of inserting these tags is to add extra sorts of information to the text:
1 a header (<head>) supplying details of the authorship & edition
2 how it should display (e.g. <bold>, <italics>)
3 what the important sections are (<h1> marks a heading, <body> is the body of the text)
4 how special symbols should display (é corresponds to é)
See also: Overview of Tags