What is the difference between README and README.md in GitHub projects?

.md is markdown. README.md is used to generate the html summary you see at the bottom of projects. Github has their own flavor of Markdown.

Order of Preference: If you have two files named README and README.md, the file named README.md is preferred, and it will be used to generate github's html summary.

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FWIW, Stack Overflow uses local Markdown modifications as well (also see Stack Overflow's C# Markdown Processor)

.md stands for markdown and is generated at the bottom of your github page as html.

Typical syntax includes:

Will become a heading
==============

Will become a sub heading
--------------

*This will be Italic*

**This will be Bold**

- This will be a list item
- This will be a list item

    Add a indent and this will end up as code

For more details: http://daringfireball.net/projects/markdown/