Our HOWTOs are released in various formats: Plain Text, HTML, PostScript, and PDF. Instead of having to write the same HOWTO in all of these formats we write just one HOWTO in a source format which gets converted by computer into all of the others.
To get an idea of what this format looks like, take a look at the source file of a webpage (if you haven't already). You will see all sorts of words in <angle brackets>. These are called tags. These webpages (tags and all) are html: Hypertext Markup Language. The LDP uses something like this for its documents.
The languages LDP uses meet the requirements of the Standard Generalized Markup Language or SGML for short. The LDP now uses the following two flavors of sgml: LinuxDoc and DocBook. Originally it only used LinuxDoc (which was sometimes incorrectly just called sgml). Interestingly, it turns out that html is still another flavor of sgml. Now some people have gone from the DocBook flavor of sgml to the DocBook flavor of the XML standard.
This mini-HOWTO is all about using the simple LinuxDoc flavor of sgml. You may call it "LinuxDoc markup". It can be machine converted to html, plain text, postscript, pdf, and DocBook. It's a lot easier than html or DocBook and you don't need a special editor for it as it's easy to type in the tags (or use macros for them).