One of the many new features of Visual Studio 2005 is the concept of solution folders.
You use them to organize the projects inside the solution, it is a logical organization because a solution folder does not map to physical directory in the file system.
For example if I create a solution folder named “DataAccess“ the following line is added to the sln file:
Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "DataAccess", "DataAccess", "{7E6B9306-943C-4E3E-99B6-043C641FF75E}"
You can do several operations on a solution folder, for example you can hide/unhide them, unload all the projects contained on a solution folder, build all the projects, etc.
Links:
http://msdn2.microsoft.com/library/haytww03(en-us,vs.80).aspx
http://msdn2.microsoft.com/library/sx2027y2(en-us,vs.80).aspx