What are the project GUIDs in a Visual Studio solution file used for?

Project persistence block in a solution file has the following format:

Project("{project type GUID}") = "<Project name>", "<project file location>", 
    "{<Unique project GUID>}"
EndProject

So it’s expected that first GUID is non-unique, it uniquely identifies Visual Studio package that handles this type of projects.

The GUID you posted is interesting – it looks like mangled C# project GUID, which is FAE04EC0-301F-11D3-BF4B-00C04F79EFBC. Did you change it yourself or is it a posting issue (e.g. you are using right-to-left locale such as Hebrew or Arabic).