Markdown is a lightweight markup language.
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 on Meta Stack Exchange -- which you can use to test out Markdown formatting.
- Markdown questions on Meta Stack Exchange
- MarkdownSharp - Server-side source code
- PageDown - Client-side source code