Key differences between old BPM and new BProc were briefly discussed in this topic.
Process modeler is integrated in the application, so you create and modify processes in the running application.
As for process forms, there are different types of process forms supported. Input dialogs process forms can be used for simple cases and can be defined in runtime. CUBA screen process forms are used in complex cases, you need to create them in IDE (CUBA Studio) and redeploy the application.
You can try the addon yourself. Get the trial at marketplace. The sampler application is available at GitHub.