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

As detailed by Oleg, the first GUID is the project type. So understandably the first GUID is not unique, but it is going to be the same based on the type of the project. Here is the list GUIDs for all common project types, courtesy of the author of this article

By Description

ASP.NET 5                                         {8BB2217D-0F2D-49D1-97BC-3654ED321F3B}
ASP.NET MVC 1                                     {603C0E0B-DB56-11DC-BE95-000D561079B0}
ASP.NET MVC 2                                     {F85E285D-A4E0-4152-9332-AB1D724D3325}
ASP.NET MVC 3                                     {E53F8FEA-EAE0-44A6-8774-FFD645390401}
ASP.NET MVC 4                                     {E3E379DF-F4C6-4180-9B81-6769533ABE47}
ASP.NET MVC 5                                     {349C5851-65DF-11DA-9384-00065B846F21}
C#                                                {FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}
C++                                               {8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}
Database                                          {A9ACE9BB-CECE-4E62-9AA4-C7E7C5BD2124}
Database (other project types)                    {4F174C21-8C12-11D0-8340-0000F80270F8}
Deployment Cab                                    {3EA9E505-35AC-4774-B492-AD1749C4943A}
Deployment Merge Module                           {06A35CCD-C46D-44D5-987B-CF40FF872267}
Deployment Setup                                  {978C614F-708E-4E1A-B201-565925725DBA}
Deployment Smart Device Cab                       {AB322303-2255-48EF-A496-5904EB18DA55}
Distributed System                                {F135691A-BF7E-435D-8960-F99683D2D49C}
Dynamics 2012 AX C# in AOT                        {BF6F8E12-879D-49E7-ADF0-5503146B24B8}
F#                                                {F2A71F9B-5D33-465A-A702-920D77279786}
J#                                                {E6FDF86B-F3D1-11D4-8576-0002A516ECE8}
Legacy (2003) Smart Device (C#)                   {20D4826A-C6FA-45DB-90F4-C717570B9F32}
Legacy (2003) Smart Device (VB.NET)               {CB4CE8C6-1BDB-4DC7-A4D3-65A1999772F8}
Micro Framework                                   {b69e3092-b931-443c-abe7-7e7b65f2a37f}
Model-View-Controller v2 (MVC 2)                  {F85E285D-A4E0-4152-9332-AB1D724D3325}
Model-View-Controller v3 (MVC 3)                  {E53F8FEA-EAE0-44A6-8774-FFD645390401}
Model-View-Controller v4 (MVC 4)                  {E3E379DF-F4C6-4180-9B81-6769533ABE47}
Model-View-Controller v5 (MVC 5)                  {349C5851-65DF-11DA-9384-00065B846F21}
Mono for Android                                  {EFBA0AD7-5A72-4C68-AF49-83D382785DCF}
MonoTouch                                         {6BC8ED88-2882-458C-8E55-DFD12B67127B}
MonoTouch Binding                                 {F5B4F3BC-B597-4E2B-B552-EF5D8A32436F}
Portable Class Library                            {786C830F-07A1-408B-BD7F-6EE04809D6DB}
Project Folders                                   {66A26720-8FB5-11D2-AA7E-00C04F688DDE}
SharePoint (C#)                                   {593B0543-81F6-4436-BA1E-4747859CAAE2}
SharePoint (VB.NET)                               {EC05E597-79D4-47f3-ADA0-324C4F7C7484}
SharePoint Workflow                               {F8810EC1-6754-47FC-A15F-DFABD2E3FA90}
Silverlight                                       {A1591282-1198-4647-A2B1-27E5FF5F6F3B}
Smart Device (C#)                                 {4D628B5B-2FBC-4AA6-8C16-197242AEB884}
Smart Device (VB.NET)                             {68B1623D-7FB9-47D8-8664-7ECEA3297D4F}
Solution Folder                                   {2150E333-8FDC-42A3-9474-1A3956D46DE8}
Test                                              {3AC096D0-A1C2-E12C-1390-A8335801FDAB}
Universal Windows Class Library                   {A5A43C5B-DE2A-4C0C-9213-0A381AF9435A}
VB.NET                                            {F184B08F-C81C-45F6-A57F-5ABD9991F28F}
Visual Database Tools                             {C252FEB5-A946-4202-B1D4-9916A0590387}
Visual Studio 2015 Installer Project Extension    {54435603-DBB4-11D2-8724-00A0C9A8B90C}
Visual Studio Tools for Applications (VSTA)       {A860303F-1F3F-4691-B57E-529FC101A107}
Visual Studio Tools for Office (VSTO)             {BAA0C2D2-18E2-41B9-852F-F413020CAA33}
Web Application                                   {349C5851-65DF-11DA-9384-00065B846F21}
Web Site                                          {E24C65DC-7377-472B-9ABA-BC803B73C61A}
Windows (C#)                                      {FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}
Windows (VB.NET)                                  {F184B08F-C81C-45F6-A57F-5ABD9991F28F}
Windows (Visual C++)                              {8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}
Windows Communication Foundation (WCF)            {3D9AD99F-2412-4246-B90B-4EAA41C64699}
Windows Phone 8/8.1 Blank/Hub/Webview App         {76F1466A-8B6D-4E39-A767-685A06062A39}
Windows Phone 8/8.1 App (C#)                      {C089C8C0-30E0-4E22-80C0-CE093F111A43}
Windows Phone 8/8.1 App (VB.NET)                  {DB03555F-0C8B-43BE-9FF9-57896B3C5E56}
Windows Presentation Foundation (WPF)             {60DC8134-EBA5-43B8-BCC9-BB4BC16C2548}
Windows Store (Metro) Apps & Components           {BC8A1FFA-BEE3-4634-8014-F334798102B3}
Workflow (C#)                                     {14822709-B5A1-4724-98CA-57A101D1B079}
Workflow (VB.NET)                                 {D59BE175-2ED0-4C54-BE3D-CDAA9F3214C8}
Workflow Foundation                               {32F31D43-81CC-4C15-9DE6-3FC5453562B6}
Xamarin.Android                                   {EFBA0AD7-5A72-4C68-AF49-83D382785DCF}
Xamarin.iOS                                       {6BC8ED88-2882-458C-8E55-DFD12B67127B}
XNA (Windows)                                     {6D335F3A-9D43-41b4-9D22-F6F17C4BE596}
XNA (XBox)                                        {2DF5C3F4-5A5F-47a9-8E94-23B4456F55E2}
XNA (Zune)                                        {D399B71A-8929-442a-A9AC-8BEC78BB2433}

By GUID

Deployment Merge Module                           {06A35CCD-C46D-44D5-987B-CF40FF872267}
Workflow (C#)                                     {14822709-B5A1-4724-98CA-57A101D1B079}
Legacy (2003) Smart Device (C#)                   {20D4826A-C6FA-45DB-90F4-C717570B9F32}
Solution Folder                                   {2150E333-8FDC-42A3-9474-1A3956D46DE8}
XNA (XBox)                                        {2DF5C3F4-5A5F-47a9-8E94-23B4456F55E2}
Workflow Foundation                               {32F31D43-81CC-4C15-9DE6-3FC5453562B6}
Web Application (incl. MVC 5)                     {349C5851-65DF-11DA-9384-00065B846F21}
Test                                              {3AC096D0-A1C2-E12C-1390-A8335801FDAB}
Windows Communication Foundation (WCF)            {3D9AD99F-2412-4246-B90B-4EAA41C64699}
Deployment Cab                                    {3EA9E505-35AC-4774-B492-AD1749C4943A}
Smart Device (C#)                                 {4D628B5B-2FBC-4AA6-8C16-197242AEB884}
Database (other project types)                    {4F174C21-8C12-11D0-8340-0000F80270F8}
Visual Studio 2015 Installer Project Extension    {54435603-DBB4-11D2-8724-00A0C9A8B90C}
SharePoint (C#)                                   {593B0543-81F6-4436-BA1E-4747859CAAE2}
ASP.NET MVC 1.0                                   {603C0E0B-DB56-11DC-BE95-000D561079B0}
Windows Presentation Foundation (WPF)             {60DC8134-EBA5-43B8-BCC9-BB4BC16C2548}
Smart Device (VB.NET)                             {68B1623D-7FB9-47D8-8664-7ECEA3297D4F}
Project Folders                                   {66A26720-8FB5-11D2-AA7E-00C04F688DDE}
MonoTouch                                         {6BC8ED88-2882-458C-8E55-DFD12B67127B}
XNA (Windows)                                     {6D335F3A-9D43-41b4-9D22-F6F17C4BE596}
Windows Phone 8/8.1 Blank/Hub/Webview App         {76F1466A-8B6D-4E39-A767-685A06062A39}
Portable Class Library                            {786C830F-07A1-408B-BD7F-6EE04809D6DB}
ASP.NET 5                                         {8BB2217D-0F2D-49D1-97BC-3654ED321F3B}
C++                                               {8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}
Deployment Setup                                  {978C614F-708E-4E1A-B201-565925725DBA}
Silverlight                                       {A1591282-1198-4647-A2B1-27E5FF5F6F3B}
Universal Windows Class Library                   {A5A43C5B-DE2A-4C0C-9213-0A381AF9435A}
Visual Studio Tools for Applications (VSTA)       {A860303F-1F3F-4691-B57E-529FC101A107}
Database                                          {A9ACE9BB-CECE-4E62-9AA4-C7E7C5BD2124}
Deployment Smart Device Cab                       {AB322303-2255-48EF-A496-5904EB18DA55}
Micro Frmework                                    {B69E3092-B931-443C-ABE7-7E7B65F2A37F}
Visual Studio Tools for Office (VSTO)             {BAA0C2D2-18E2-41B9-852F-F413020CAA33}
Windows Store Apps (Metro Apps)                   {BC8A1FFA-BEE3-4634-8014-F334798102B3}
C# in Dynamics 2012 AX AOT                        {BF6F8E12-879D-49E7-ADF0-5503146B24B8}
Windows Phone 8/8.1 App (C#)                      {C089C8C0-30E0-4E22-80C0-CE093F111A43}
Visual Database Tools                             {C252FEB5-A946-4202-B1D4-9916A0590387}
Legacy (2003) Smart Device (VB.NET)               {CB4CE8C6-1BDB-4DC7-A4D3-65A1999772F8}
XNA (Zune)                                        {D399B71A-8929-442a-A9AC-8BEC78BB2433}
Workflow (VB.NET)                                 {D59BE175-2ED0-4C54-BE3D-CDAA9F3214C8}
Windows Phone 8/8.1 App (VB.NET)                  {DB03555F-0C8B-43BE-9FF9-57896B3C5E56}
Web Site                                          {E24C65DC-7377-472B-9ABA-BC803B73C61A}
ASP.NET MVC 4.0                                   {E3E379DF-F4C6-4180-9B81-6769533ABE47}
ASP.NET MVC 3.0                                   {E53F8FEA-EAE0-44A6-8774-FFD645390401}
J#                                                {E6FDF86B-F3D1-11D4-8576-0002A516ECE8}
SharePoint (VB.NET)                               {EC05E597-79D4-47f3-ADA0-324C4F7C7484}
Xamarin.Android / Mono for Android                {EFBA0AD7-5A72-4C68-AF49-83D382785DCF}
Distributed System                                {F135691A-BF7E-435D-8960-F99683D2D49C}
VB.NET                                            {F184B08F-C81C-45F6-A57F-5ABD9991F28F}
F#                                                {F2A71F9B-5D33-465A-A702-920D77279786}
MonoTouch Binding                                 {F5B4F3BC-B597-4E2B-B552-EF5D8A32436F}
ASP.NET MVC 2.0                                   {F85E285D-A4E0-4152-9332-AB1D724D3325}
SharePoint Workflow                               {F8810EC1-6754-47FC-A15F-DFABD2E3FA90}
C#                                                {FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}