The Markdown Editor for Umbraco is a data-type that enables you to write your content using Markdown syntax. Configuration options are available to enable/disable the WMD Editor (powered by MarkEdit).
http://our.umbraco.org/projects/backoffice-extensions/markdown-editor
Markdown is a text-to-HTML conversion tool for web writers. Markdown allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML). Further information here: http://daringfireball.net/projects/markdown/
WMD is a simple, lightweight HTML editor for basic content management. The WMD editor became extremely popular due to its use on the StackOverflow website. The original WMD Editor website: http://wmd-editor.com/
MarkEdit is a fork/re-write of the WMD for Stackoverflow. It has extensive improvements and modifications which implement customization, a public api, configurability, and internationalization. MarkEdit website: http://tstone.github.com/jquery-markedit/