Markdown is a lightweight, plain-text, markup language. It is used across the entire Stack Exchange network for post formatting (Questions, answers, wikis, chat).
Markdown is a lightweight markup language used across the entire Stack Exchange network for post formatting. The language takes many cues from existing conventions for marking up plain text in email; it is designed for maximum readability and "publishability" of both its input and output forms.
The code used in the Markdown is open source and can be found at the following:
If you'd like to test out Markdown formatting, you can use the Formatting Sandbox.