In a .csproj file, what is <None Include="..."> for?

One difference is how they get published; "None" items don't get included in a publish, "Content" items do; for example, on the "Application Files" dialog on the Publish tab.

The MSDN article on the build action property explains the differences.

None - The file is not included in the project output group and is not compiled in the build process. An example is a text file that contains documentation, such as a Readme file.

Content - The file is not compiled, but is included in the Content output group. For example, this setting is the default value for an .htm or other kind of Web file.

I am not 100% sure (I read the MSDN description of Build Action property) but just copying that answer from MSDN to StackOverflow does not answer the question completely for me.

The difference of None and Content only has an effect on Web projects. For a command line project, WinForm project or UnitTest project (in my case) etc. None and Content have no different behavior.

MSDN: "project output group" or "Content output group" only terms used in a Web project, right?