Standard Generalised Mark-up Language, or SGML, is a language to define document types.
For instance, one may define the document type recipe, with a first part presenting the ingredients, a second part introducing the accessories, a third part giving step by step instructions for baking the cake, and a nice final picture to show the outcome of it all.
This is called a Document Type Definition. It does not define what the final product will look like, it only defines what it may contain.
To use the same example again, I'm sure that upon reading my idea of a recipe, you recognised yours, or your favourite cook's. Nevertheless, they actually look different: mine have a picture in the upper left corner of the bathroom cupboard, and the ingredients list can be found in the back garden, between the swimming pool and the barbecue. Yours?
Thanks to this standard definition, one can write a document, without taking into account what it will look like in the end to the reader.
This type is used to write, as you might have guessed, documents related to Linux.
Such documents are generally built as follows: they start with a title followed by the name of the author, and the version number and date. Then comes the abstract (so you don't have to browse through it before realizing it isn't what you were looking for after all), then the contents which show the structure so that those in a rush can go directly to the part they want to read.
Then comes a list of chapters, sections, paragraphs. Among these, one can insert bits of programs, change the font to emphasise a word or a sentence, insert lists, refer to another part of the document, etc.
To write such a document, you just need to specify at the right time the title, the author, the date, and the document version, the chapters and sections, say when a list is to be inserted, what its elements are etc.
SGML-Tools will turn the specification of a document into the final result in the form you prefer. If you want it in your personal library, you will choose PostScript. If you want to share it with the world through the Web, it will be HTML. If you can't help it and must read it under Windows, you can turn it into RTF to be able to read it with any word processor. Or maybe use all three formats to accommodate your changing moods.
SGML-Tools are available via anonymous FTP at ftp://ftp.lip6.fr/pub/sgml-tools/