You can think of the "solution" as everything needed to build ONE complete application. For instance, if you are building an app called MyApp, and MyApp uses three DLLs and one library that you also created, your solution would contain fiveProjects:
MyApp
Dll1
Dll2
Dll3
Lib

You also then set the Project Dependencies so they are build in the correct order. For instance, if DLL3 requires something from the Lib, and DLL2 requires DLL3, and MyApp requires all of them, you would set the dependencies to indicate that. This guarantees that if you change something in the lib, the lib would be built first, then DLL3 would be re-built as needed, then DLL2, then MyApp.

As for "extra" projects that show in the solution explorer, you can simply delete them from the solution explorer - that removes them from the solution but does not delete the files on your disk.

Hope that helps.