From Seo Wiki - Search Engine Optimization and Programming Languages

A lightweight markup language is a markup language with a simple syntax, designed to be easy for a human to enter with a simple text editor, and easy to read in its raw form.

Lightweight markup languages are used in applications where people might be expected to read the document source as well as the rendered output. For instance, a person downloading a software library might prefer to read the documentation in a text editor rather than a browser.

History

Lightweight markup languages were originally used on text-only displays which could not display characters in italics or bold, so informal methods to convey this information had to be developed. This formatting choice was naturally carried forth to plain-text email communications.

1986 international standard SGML provided facilities to define and parse lightweight markup languages using grammars and tag implication. The 1998 W3C XML is a profile of SGML that omits these facilities.

Any of the following characters can be used as the "underline": = - ` : ' " ~ ^ _ * + # < >. The same character must be used for the same indentation level and may not be used for a new indentation level.

You can optionally "close" the #-style headers with an equal number of #'s. The #-style headers support up to 6 levels. The underlined style headers support four levels (### *** === ----). Resulting absolute levels numbers are calculated dynamically. Optionally, less chars can denote higher header level.