If you've added Q_OBJECT to a self-defined class you need to run qmake prior to compilation, otherwise it won't generated the meta code needed for it.
1) Not sure what you mean here, sorry.
2) Or here.
3) The .pro and .pri are essentially directives files for qmake, much the same way Makefiles work for make. There's obvious differences, but you're not far off when you say they're for mapping, they're there to tell qmake how to generate the necessary compilation files. When you're adding new classes if they've not been auto-added then you need to add both the .cpp & .h files.