Markup languages are sets of information designed to perform specific functions within an electronic document. Standard Generalized Markup Language specifies how markup languages work. Things that describe the 'big picture' of how things work are often given a name that is prefixed with meta. Since SGML specifies how markup languages work, SGML is called a metalanguage.
Computer documents (electronic texts) contain a title, heading, body, paragraphs, lists, word definitions, footings and more. These documents also contain formatting such as italics, bold, underline, font sizes and colors. All this information has to be encoded into the document in some fashion so that it is retained when the document is stored and later re-opened. This formatting information needs to be cross-platform so that a document created on one computer can be read and properly displayed on any other computer. This makes it possible for a document to be created on a MacIntosh computer and still contain the same formatting when read on a Windows computer.
SGML languages use markers called tags to specify the formatting within the document and to separate specific parts of the document into blocks that can be recognized and searched more easilly. SGML specifies what the tags look like and what the tags do. The specification for a specific markup language is contained within the Document Type Definition (DTD). A computer can read the DTD and from it extrapolate what the tags embedded in the document do. It is then up to the computer to properly display the information as specified in the DTD and the document.
Two of the most well-known forms of SGML are HTML and XML/XHTML. The focus of SGML is to make the tags describe what they do instead of actually having the tags do the formatting themselves. To turn text bold, you could use the function built into in the hardware and stick the command for that function into the document, but then the document would be unreadable on any other type of computer. That's why SGML only describes what should be done, it doesn't actually do any of the formatting.
This page is a placeholder for a future tutorial, perhaps on SGML tags and the DTD.