title: What are the project GUIDs in a Visual Studio solution file used for?
body: <p>I have multiple projects in a single Visual Studio (2008) solution.<br>
  I just discovered that each of these projects uses a same GUID, so in the solution file it looks like this:</p>
  
  <pre><code>Project("{FAE04EC0-F103-D311-BF4B-00C04FCBFE97}") = "Pro1", "Pro1\Pro1.csproj", "{...}"
  Project("{FAE04EC0-F103-D311-BF4B-00C04FCBFE97}") = "Pro2", "Pro2\Pro2.csproj", "{...}"
  </code></pre>
  
  <p>Do I have to change these GUIDs so they're unique and what are they used for?</p>