During the git-svn study, I notice there are lots of document in github are in markdown format, and showing in html looks very concise like wiki page.
Wikipedia gives excellent definition for markdown, see http://en.wikipedia.org/wiki/Markdown
Markdown is a lightweight markup language, originally created by John Gruber and Aaron Swartz to help maximum readability and "publishability" of both its input and output forms. The language takes many cues from existing conventions for marking up plain text in email.
Let’s compare it to other my used document format
Compare to M$ Word, it is plain text, easy to write and controlled by version control system, and best of all, it lets me focus on content instead of format
Compare to Wiki, the format is quite similar, and markdown is a file, it means, I can easily put into svn/git and publish in other place, wiki system force in most of time to write online
Compare to Docbook (http://www.docbook.org/) it is a plain text instead of xml format, it can help me to focus on the contents I am writting and reach to “publishability”, since it is more readability from the source, for docbook, mostly I need to generate html to see what it looks like, or rely on my software like
Compare to apt (http://maven.apache.org/doxia/references/apt-format.html), the format is quite similar, markdown seems more popular and well acceptable by community and it is better for publishability.
And always remind myself, “how to write the information is more important than choosing tools” 
