ITPForm and ITPSubmitButton respectively represent the HTML form and the buttons that submit this form. They both add a submit function, which will submit the form, passing information on the button that was clicked to submit the form.
ITPGroup represents a group on the page. It implements behavior for expanding, collapsing, and toggling of groups. The latter is implemented by processing messages that are sent by question elements.
ITPQuestion represents a question on the form. Its descendants define the specific behavior for different question types. ITPSelectQuestion and its descendants define the behavior for select questions; ITPSingleSelectQuestion for single selects and ITPMultiSelectQuestion for multi selects.