Markdown is a markup language which can allow for plain-text documents to be transformed into HTML and other formats.
Markdown was developed by John Gruber, and described on his website. However, the grammar was not rigorously specified, leading to a number of variant implementations. In September 2014, a Standard Markdown specification was released to address these concerns.
Some implementations of Markdown also offer non-standard extensions to the language. For example, Stack Exchange uses an enhanced version of Markdown to allow editing of Questions and Answers, and even this wiki text. For help on the Stack Exchange enhancements check out the help pages.