Markdown is a lightweight markup language. It originated as a text to HTML conversion in Perl, done as a project at Darling Fireball. It’s since been reimplemented in lots of languages, and extended half a dozen ways.
Under the covers, most of this is written in Markdown, and converted using Maruku via Jekyll.
The Markdown page at Darling Fireball is the “official” docs.
Many implementations support the extensions in php markdown extra
Maruku supports basic syntax, markdown extra, as well as it’s own metadata syntax