A Solution (.sln) is a container for one or more projects. Visual Studio requires a Solution before you build anything. A project builds one thing in one language - .vfproj is the file type for Intel Visual Fortran projects. If you open a project that implicitly opens the solution it is in. In the simple case, you'll have one solution with one executable project, but you could have multiple projects of different types (exe, dll, lib, etc.) and/or different languages (Fortran, C++, VB, etc.) in a solution. You can establish a tree of dependencies among projects in a solution so that VS builds them in the proper order, and automatically include the output of a child project (say, a library) in the build of its parent.
