.NET: SGML in brief

SGML was developed and standardised by the International Organisation for Standards (ISO).
SGML is a system or meta-language for defining markup languages, organising and tagging elements of a document. Authors markup their documents by representing structural, presentational, and semantic information alongside content. Each markup language defined in SGML is called an SGML application. HTML and XML are based on SGML.

An SGML application is generally characterized by:

An SGML declaration.
2. A document type definition (DTD).
3. A specification that describes the semantics to be ascribed to the markup.
Document instances containing data (content) and markup.