SGML stands for the Standard Generalized Markup Language. SGML is defined in ISO 8879:1986 "Information Processing -- Text and Office Systems -- Standard Generalized Markup Language (SGML)". A copy of this document can be obtained from the International Organization for Standardization (ISO) or your national standard body. It is not available for ftp.
SGML enables the description of structured information _independent_ of how that information is processed. It is a meta-language that provides a standard syntax for defining descriptions of classes of structured information; these descriptions are called document type definitions (DTDs). Information can be "marked up" according to a DTD, so that its structure is made explicit and accessible. The "markup" can be checked against a DTD to ensure that it is valid, and thus that the structure of the information conforms to that of the class described by the DTD. Ensuring that information is structured in a known way greatly facilitates any subsequent use of that information. For more information, beginners should read Erik Naggum's "Introductory questions with answers" , consult ISO 8879 and/or a book on SGML.
For more introductory information look at this SGML F.A.Q.