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

Question

I have multiple projects in a single Visual Studio (2008) solution.
I just discovered that each of these projects uses a same GUID, so in the solution file it looks like

Answer 1

The project guids in the csproj file should be unique. From my experience if two csproj files are included in the same solution visual studio automatically regenerates one of the guids.