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Usage guidance, also known as a tag wiki excerpt, is a short blurb that describes when and why a tag should be used on this site specifically.
The standard editor used to enter Stack Exchange questions, answers, wiki entries, user profile about me field, and so forth. It supports a live preview of Markdown syntax.
This editor is open source.
On the server it is MarkdownSharp: http://code.google.com/p/markdownsharp/
On the client it is PageDown: http://code.google.com/p/pagedown/