Standard generalized markup language (SGML) is a mechanism for marking up text. The markup describes the text. How does it describe the text?
There are three ways that SGML can describe the marked up text:
PRESENTATION - You can use SGML markup to describe the presentation (i.e. style, format) of the text. For example: <font family="courier" size="10">These words</font> might instruct an SGML processor that "These words" should presented (formatted) in a 10 pt courier font.
STRUCTURE - You can use SGML markup to describe the structure of your text. For example: <p>This is a paragraph</p> might instruct an SGML processor to treat the text as a paragraph.
CONTENT - You can use SGML markup to describe the content of your text. For example: <phone-number>555-1212</phone-number> might instruct an SGML processor to treat the text as a phone number.