There is no such thing as a standard file type named as *.proj and associated with Visual Studio. Many different project files (but not all of them) are based on the same format (MSBuild, described in documentation for MSBuild), but they are named based on the language, such as *.csproj, *.vbproj, etc.
Normally, they are created when you add a new project in Visual Studio from on or another available
project template. However, a more skillful developer can create them manually or using Visual Studio XML editor (or some other XML editor), can write some utilities manipulating such files using MSBuild API or not, and so on. In particular, you can define your own file type with appropriate naming scheme (such as *.proj), register it in your system and use for building project, solution, incorporating 3rd-party tools and other advanced building techniques.
If you really want to delve into it (are you sure you do and will be able to?), you can learn the subject starting from:
http://msdn.microsoft.com/en-us/library/wea2sca5%28v=vs.90%29.aspx[
^],
http://msdn.microsoft.com/en-us/library/0k6kkbsd.aspx[
^].
—SA
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