How Does SGML Work?

SGML is a simple, text-based annotation (or tagging) standard used to describe the contents of a text or multimedia document. An SGML document consists of content (information) and annotation (markup).
One part of a typical SGML document may contain basic text, another part may contain a specific kind of text, and yet another part may refer to a video file, and there are annotations (""). The annotations describe what the information is, not what it should look like. The content can be any form of electronic information.
An SGML file, then, is a collection of information with tags that describe that information.

Follow the Rules

While the information in an SGML document may be described with SGML annotations (markup), it may only be described according to a specific set of possible tags. Every SGML document must be accompanied by a kind of legend or key which explains the mark-up used in that document.
Each individual legend, or set of predefined SGML mark-up, is known as a Document Type Definition (DTD) or a Rules File.
Different DTDs may be created for different types of documents, but each individual document must follow only its own DTD. An SGML DTD outlines all the possible markup for particular class of documents and specifies where the different mark-up can go; An SGML document is made up of electronic information described by some or all of the types of mark-up defined in the corresponding DTD.

How Does SGML relate to HTML?

If some of the markup above looked a lot like HTML, that's because HTML is SGML.
HTML is actually one set of predefined SGML markup: it's a DTD. It simply happens to be such a widely accepted DTD that it is sometimes confused as a language itself. HTML files are collections of information with markup describing that information. HTML browsers such as Netscape and Mosaic read the markup in HTML files and decide how to display the appropriate information based on that mark-up.
Browsers, though, need not be limited to HTML. Different firms produce many full SGML browsers that can display any DTD (including HTML).

How Do I Start Using SGML?

You'll need the following things to get started with SGML:

Electronic Information
You should have information you want to access in a variety of ways (print, CD-ROM, on the Web) or a need to manage your information efficiently. Compare the following list with your information needs; SGML offers a solution for these situations:
+ I'd like my information to be easily exchanged between different computers and software.
+ Users of my information should be able to select the information they want in very sophisticated ways.
+ I want the same information to be presented in whatever publishing format best suits the needs of the user.
+ My information needs to be long lived: I want to reuse it with many applications, over time and extend its shelf-life. DTDs For each type of document you want to publish or view, you'll need a separate Document Type Definition (DTD) (you won't need a different DTD for separate instances of the same type of document) You can write DTDs yourself, have a consultant write them for you, or use an existing DTD.