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. 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.
It is used across the entire Stack Exchange network for formatting. Markdown is used for: Questions, Answers, Wikis, Chat and Comments -- although not all features are enabled in all contexts.
- Markdown help
- How do I format my posts using Markdown or HTML?
- The Formatting Sandbox -- which you can use to test out Markdown formatting.
- MarkdownSharp - Server-side source code
- PageDown - Client-side source code
Copied from: http://meta.stackexchange.com/tags/markdown/info